essay scoring system

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26 papers with code • 1 benchmarks • 1 datasets

Essay scoring: Automated Essay Scoring is the task of assigning a score to an essay, usually in the context of assessing the language ability of a language learner. The quality of an essay is affected by the following four primary dimensions: topic relevance, organization and coherence, word usage and sentence complexity, and grammar and mechanics.

Source: A Joint Model for Multimodal Document Quality Assessment

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Most implemented papers

Automated essay scoring based on two-stage learning.

Current state-of-art feature-engineered and end-to-end Automated Essay Score (AES) methods are proven to be unable to detect adversarial samples, e. g. the essays composed of permuted sentences and the prompt-irrelevant essays.

A Neural Approach to Automated Essay Scoring

nusnlp/nea • EMNLP 2016

SkipFlow: Incorporating Neural Coherence Features for End-to-End Automatic Text Scoring

Our new method proposes a new \textsc{SkipFlow} mechanism that models relationships between snapshots of the hidden representations of a long short-term memory (LSTM) network as it reads.

Neural Automated Essay Scoring and Coherence Modeling for Adversarially Crafted Input

Youmna-H/Coherence_AES • NAACL 2018

We demonstrate that current state-of-the-art approaches to Automated Essay Scoring (AES) are not well-suited to capturing adversarially crafted input of grammatical but incoherent sequences of sentences.

Co-Attention Based Neural Network for Source-Dependent Essay Scoring

This paper presents an investigation of using a co-attention based neural network for source-dependent essay scoring.

Language models and Automated Essay Scoring

In this paper, we present a new comparative study on automatic essay scoring (AES).

Evaluation Toolkit For Robustness Testing Of Automatic Essay Scoring Systems

midas-research/calling-out-bluff • 14 Jul 2020

This number is increasing further due to COVID-19 and the associated automation of education and testing.

Prompt Agnostic Essay Scorer: A Domain Generalization Approach to Cross-prompt Automated Essay Scoring

Cross-prompt automated essay scoring (AES) requires the system to use non target-prompt essays to award scores to a target-prompt essay.

Many Hands Make Light Work: Using Essay Traits to Automatically Score Essays

To find out which traits work best for different types of essays, we conduct ablation tests for each of the essay traits.

EXPATS: A Toolkit for Explainable Automated Text Scoring

octanove/expats • 7 Apr 2021

Automated text scoring (ATS) tasks, such as automated essay scoring and readability assessment, are important educational applications of natural language processing.

ORIGINAL RESEARCH article

Explainable automated essay scoring: deep learning really has pedagogical value.

• School of Computing and Information Systems, Faculty of Science and Technology, Athabasca University, Edmonton, AB, Canada

Automated essay scoring (AES) is a compelling topic in Learning Analytics for the primary reason that recent advances in AI find it as a good testbed to explore artificial supplementation of human creativity. However, a vast swath of research tackles AES only holistically; few have even developed AES models at the rubric level, the very first layer of explanation underlying the prediction of holistic scores. Consequently, the AES black box has remained impenetrable. Although several algorithms from Explainable Artificial Intelligence have recently been published, no research has yet investigated the role that these explanation models can play in: (a) discovering the decision-making process that drives AES, (b) fine-tuning predictive models to improve generalizability and interpretability, and (c) providing personalized, formative, and fine-grained feedback to students during the writing process. Building on previous studies where models were trained to predict both the holistic and rubric scores of essays, using the Automated Student Assessment Prize’s essay datasets, this study focuses on predicting the quality of the writing style of Grade-7 essays and exposes the decision processes that lead to these predictions. In doing so, it evaluates the impact of deep learning (multi-layer perceptron neural networks) on the performance of AES. It has been found that the effect of deep learning can be best viewed when assessing the trustworthiness of explanation models. As more hidden layers were added to the neural network, the descriptive accuracy increased by about 10%. This study shows that faster (up to three orders of magnitude) SHAP implementations are as accurate as the slower model-agnostic one. It leverages the state-of-the-art in natural language processing, applying feature selection on a pool of 1592 linguistic indices that measure aspects of text cohesion, lexical diversity, lexical sophistication, and syntactic sophistication and complexity. In addition to the list of most globally important features, this study reports (a) a list of features that are important for a specific essay (locally), (b) a range of values for each feature that contribute to higher or lower rubric scores, and (c) a model that allows to quantify the impact of the implementation of formative feedback.

Automated essay scoring (AES) is a compelling topic in Learning Analytics (LA) for the primary reason that recent advances in AI find it as a good testbed to explore artificial supplementation of human creativity. However, a vast swath of research tackles AES only holistically; only a few have even developed AES models at the rubric level, the very first layer of explanation underlying the prediction of holistic scores ( Kumar et al., 2017 ; Taghipour, 2017 ; Kumar and Boulanger, 2020 ). None has attempted to explain the whole decision process of AES, from holistic scores to rubric scores and from rubric scores to writing feature modeling. Although several algorithms from XAI (explainable artificial intelligence) ( Adadi and Berrada, 2018 ; Murdoch et al., 2019 ) have recently been published (e.g., LIME, SHAP) ( Ribeiro et al., 2016 ; Lundberg and Lee, 2017 ), no research has yet investigated the role that these explanation models (trained on top of predictive models) can play in: (a) discovering the decision-making process that drives AES, (b) fine-tuning predictive models to improve generalizability and interpretability, and (c) providing teachers and students with personalized, formative, and fine-grained feedback during the writing process.

One of the key anticipated benefits of AES is the elimination of human bias such as rater fatigue, rater’s expertise, severity/leniency, scale shrinkage, stereotyping, Halo effect, rater drift, perception difference, and inconsistency ( Taghipour, 2017 ). At its turn, AES may suffer from its own set of biases (e.g., imperfections in training data, spurious correlations, overrepresented minority groups), which has incited the research community to look for ways to make AES more transparent, accountable, fair, unbiased, and consequently trustworthy while remaining accurate. This required changing the perception that AES is merely a machine learning and feature engineering task ( Madnani et al., 2017 ; Madnani and Cahill, 2018 ). Hence, researchers have advocated that AES should be seen as a shared task requiring several methodological design decisions along the way such as curriculum alignment, construction of training corpora, reliable scoring process, and rater performance evaluation, where the goal is to build and deploy fair and unbiased scoring models to be used in large-scale assessments and classroom settings ( Rupp, 2018 ; West-Smith et al., 2018 ; Rupp et al., 2019 ). Unfortunately, although these measures are intended to design reliable and valid AES systems, they may still fail to build trust among users, keeping the AES black box impenetrable for teachers and students.

It has been previously recognized that divergence of opinion among human and machine graders has been only investigated superficially ( Reinertsen, 2018 ). So far, researchers investigated the characteristics of essays through qualitative analyses which ended up rejected by AES systems (requiring a human to score them) ( Reinertsen, 2018 ). Others strived to justify predicted scores by identifying essay segments that actually caused the predicted scores. In spite of the fact that these justifications hinted at and quantified the importance of these spatial cues, they did not provide any feedback as to how to improve those suboptimal essay segments ( Mizumoto et al., 2019 ).

Related to this study and the work of Kumar and Boulanger (2020) is Revision Assistant, a commercial AES system developed by Turnitin ( Woods et al., 2017 ; West-Smith et al., 2018 ), which in addition to predicting essays’ holistic scores provides formative, rubric-specific, and sentence-level feedback over multiple drafts of a student’s essay. The implementation of Revision Assistant moved away from the traditional approach to AES, which consists in using a limited set of features engineered by human experts representing only high-level characteristics of essays. Like this study, it rather opted for including a large number of low-level writing features, demonstrating that expert-designed features are not required to produce interpretable predictions. Revision Assistant’s performance was reported on two essay datasets, one of which was the Automated Student Assessment Prize (ASAP) 1 dataset. However, performance on the ASAP dataset was reported in terms of quadratic weighted kappa and this for holistic scores only. Models predicting rubric scores were trained only with the other dataset which was hosted on and collected through Revision Assistant itself.

In contrast to feature-based approaches like the one adopted by Revision Assistant, other AES systems are implemented using deep neural networks where features are learned during model training. For example, Taghipour (2017) in his doctoral dissertation leverages a recurrent neural network to improve accuracy in predicting holistic scores, implement rubric scoring (i.e., organization and argument strength), and distinguish between human-written and computer-generated essays. Interestingly, Taghipour compared the performance of his AES system against other AES systems using the ASAP corpora, but he did not use the ASAP corpora when it came to train rubric scoring models although ASAP provides two corpora provisioning rubric scores (#7 and #8). Finally, research was also undertaken to assess the generalizability of rubric-based models by performing experiments across various datasets. It was found that the predictive power of such rubric-based models was related to how much the underlying feature set covered a rubric’s criteria ( Rahimi et al., 2017 ).

Despite their numbers, rubrics (e.g., organization, prompt adherence, argument strength, essay length, conventions, word choices, readability, coherence, sentence fluency, style, audience, ideas) are usually investigated in isolation and not as a whole, with the exception of Revision Assistant which provides feedback at the same time on the following five rubrics: claim, development, audience, cohesion, and conventions. The literature reveals that rubric-specific automated feedback includes numerical rubric scores as well as recommendations on how to improve essay quality and correct errors ( Taghipour, 2017 ). Again, except for Revision Assistant which undertook a holistic approach to AES including holistic and rubric scoring and provision of rubric-specific feedback at the sentence level, AES has generally not been investigated as a whole or as an end-to-end product. Hence, the AES used in this study and developed by Kumar and Boulanger (2020) is unique in that it uses both deep learning (multi-layer perceptron neural network) and a huge pool of linguistic indices (1592), predicts both holistic and rubric scores, explaining holistic scores in terms of rubric scores, and reports which linguistic indices are the most important by rubric. This study, however, goes one step further and showcases how to explain the decision process behind the prediction of a rubric score for a specific essay, one of the main AES limitations identified in the literature ( Taghipour, 2017 ) that this research intends to address, at least partially.

Besides providing explanations of predictions both globally and individually, this study not only goes one step further toward the automated provision of formative feedback but also does so in alignment with the explanation model and the predictive model, allowing to better map feedback to the actual characteristics of an essay. Woods et al. (2017) succeeded in associating sentence-level expert-derived feedback with strong/weak sentences having the greatest influence on a rubric score based on the rubric, essay score, and the sentence characteristics. While Revision Assistant’s feature space consists of counts and binary occurrence indicators of word unigrams, bigrams and trigrams, character four-grams, and part-of-speech bigrams and trigrams, they are mainly textual and locational indices; by nature they are not descriptive or self-explanative. This research fills this gap by proposing feedback based on a set of linguistic indices that can encompass several sentences at a time. However, the proposed approach omits locational hints, leaving the merging of the two approaches as the next step to be addressed by the research community.

Although this paper proposes to extend the automated provision of formative feedback through an interpretable machine learning method, it rather focuses on the feasibility of automating it in the context of AES instead of evaluating the pedagogical quality (such as the informational and communicational value of feedback messages) or impact on students’ writing performance, a topic that will be kept for an upcoming study. Having an AES system that is capable of delivering real-time formative feedback sets the stage to investigate (1) when feedback is effective, (2) the types of feedback that are effective, and (3) whether there exist different kinds of behaviors in terms of seeking and using feedback ( Goldin et al., 2017 ). Finally, this paper omits describing the mapping between the AES model’s linguistic indices and a pedagogical language that is easily understandable by students and teachers, which is beyond its scope.

Methodology

This study showcases the application of the PDR framework ( Murdoch et al., 2019 ), which provides three pillars to describe interpretations in the context of the data science life cycle: P redictive accuracy, D escriptive accuracy, and R elevancy to human audience(s). It is important to note that in a broader sense both terms “explainable artificial intelligence” and “interpretable machine learning” can be used interchangeably with the following meaning ( Murdoch et al., 2019 ): “the use of machine-learning models for the extraction of relevant knowledge about domain relationships contained in data.” Here “predictive accuracy” refers to the measurement of a model’s ability to fit data; “descriptive accuracy” is the degree at which the relationships learned by a machine learning model can be objectively captured; and “relevant knowledge” implies that a particular audience gets insights into a chosen domain problem that guide its communication, actions, and discovery ( Murdoch et al., 2019 ).

In the context of this article, formative feedback that assesses students’ writing skills and prescribes remedial writing strategies is the relevant knowledge sought for, whose effectiveness on students’ writing performance will be validated in an upcoming study. However, the current study puts forward the tools and evaluates the feasibility to offer this real-time formative feedback. It also measures the predictive and descriptive accuracies of AES and explanation models, two key components to generate trustworthy interpretations ( Murdoch et al., 2019 ). Naturally, the provision of formative feedback is dependent on the speed of training and evaluating new explanation models every time a new essay is ingested by the AES system. That is why this paper investigates the potential of various SHAP implementations for speed optimization without compromising the predictive and descriptive accuracies. This article will show how the insights generated by the explanation model can serve to debug the predictive model and contribute to enhance the feature selection and/or engineering process ( Murdoch et al., 2019 ), laying the foundation for the provision of actionable and impactful pieces of knowledge to educational audiences, whose relevancy will be judged by the human stakeholders and estimated by the magnitude of resulting changes.

Figure 1 overviews all the elements and steps encompassed by the AES system in this study. The following subsections will address each facet of the overall methodology, from hyperparameter optimization to relevancy to both students and teachers.

Figure 1. A flow chart exhibiting the sequence of activities to develop an end-to-end AES system and how the various elements work together to produce relevant knowledge to the intended stakeholders.

Automated Essay Scoring System, Dataset, and Feature Selection

As previously mentioned, this paper reuses the AES system developed by Kumar and Boulanger (2020) . The AES models were trained using the ASAP’s seventh essay corpus. These narrative essays were written by Grade-7 students in the setting of state-wide assessments in the United States and had an average length of 171 words. Students were asked to write a story about patience. Kumar and Boulanger’s work consisted in training a predictive model for each of the four rubrics according to which essays were graded: ideas, organization, style, and conventions. Each essay was scored by two human raters on a 0−3 scale (integer scale). Rubric scores were resolved by adding the rubric scores assigned by the two human raters, producing a resolved rubric score between 0 and 6. This paper is a continuation of Boulanger and Kumar (2018 , 2019 , 2020) and Kumar and Boulanger (2020) where the objective is to open the AES black box to explain the holistic and rubric scores that it predicts. Essentially, the holistic score ( Boulanger and Kumar, 2018 , 2019 ) is determined and justified through its four rubrics. Rubric scores, in turn, are investigated to highlight the writing features that play an important role within each rubric ( Kumar and Boulanger, 2020 ). Finally, beyond global feature importance, it is not only indispensable to identify which writing indices are important for a particular essay (local), but also to discover how they contribute to increase or decrease the predicted rubric score, and which feature values are more/less desirable ( Boulanger and Kumar, 2020 ). This paper is a continuation of these previous works by adding the following link to the AES chain: holistic score, rubric scores, feature importance, explanations, and formative feedback. The objective is to highlight the means for transparent and trustable AES while empowering learning analytics practitioners with the tools to debug these models and equip educational stakeholders with an AI companion that will semi-autonomously generate formative feedback to teachers and students. Specifically, this paper analyzes the AES reasoning underlying its assessment of the “style” rubric, which looks for command of language, including effective and compelling word choice and varied sentence structure, that clearly supports the writer’s purpose and audience.

This research’s approach to AES leverages a feature-based multi-layer perceptron (MLP) deep neural network to predict rubric scores. The AES system is fed by 1592 linguistic indices quantitatively measured by the Suite of Automatic Linguistic Analysis Tools 2 (SALAT), which assess aspects of grammar and mechanics, sentiment analysis and cognition, text cohesion, lexical diversity, lexical sophistication, and syntactic sophistication and complexity ( Kumar and Boulanger, 2020 ). The purpose of using such a huge pool of low-level writing features is to let deep learning extract the most important ones; the literature supports this practice since there is evidence that features automatically selected are not less interpretable than those engineered ( Woods et al., 2017 ). However, to facilitate this process, this study opted for a semi-automatic strategy that consisted of both filter and embedded methods. Firstly, the original ASAP’s seventh essay dataset consists of a training set of 1567 essays and a validation and testing sets of 894 essays combined. While the texts of all 2461 essays are still available to the public, only the labels (the rubric scores of two human raters) of the training set have been shared with the public. Yet, this paper reused the unlabeled 894 essays of the validation and testing sets for feature selection, a process that must be carefully carried out by avoiding being informed by essays that will train the predictive model. Secondly, feature data were normalized, and features with variances lower than 0.01 were pruned. Thirdly, the last feature of any pair of features having an absolute Pearson correlation coefficient greater than 0.7 was also pruned (the one that comes last in terms of the column ordering in the datasets). After the application of these filter methods, the number of features was reduced from 1592 to 282. Finally, the Lasso and Ridge regression regularization methods (whose combination is also called ElasticNet) were applied during the training of the rubric scoring models. Lasso is responsible for pruning further features, while Ridge regression is entrusted with eliminating multicollinearity among features.

Hyperparameter Optimization and Training

To ensure a fair evaluation of the potential of deep learning, it is of utmost importance to minimally describe this study’s exploration of the hyperparameter space, a step that is often found to be missing when reporting the outcomes of AES models’ performance ( Kumar and Boulanger, 2020 ). First, a study should list the hyperparameters it is going to investigate by testing for various values of each hyperparameter. For example, Table 1 lists all hyperparameters explored in this study. Note that L 1 and L 2 are two regularization hyperparameters contributing to feature selection. Second, each study should also report the range of values of each hyperparameter. Finally, the strategy to explore the selected hyperparameter subspace should be clearly defined. For instance, given the availability of high-performance computing resources and the time/cost of training AES models, one might favor performing a grid (a systematic testing of all combinations of hyperparameters and hyperparameter values within a subspace) or a random search (randomly selecting a hyperparameter value from a range of values per hyperparameter) or both by first applying random search to identify a good starting candidate and then grid search to test all possible combinations in the vicinity of the starting candidate’s subspace. Of particular interest to this study is the neural network itself, that is, how many hidden layers should a neural network have and how many neurons should compose each hidden layer and the neural network as a whole. These two variables are directly related to the size of the neural network, with the number of hidden layers being a defining trait of deep learning. A vast swath of literature is silent about the application of interpretable machine learning in AES and even more about measuring its descriptive accuracy, the two components of trustworthiness. Hence, this study pioneers the comprehensive assessment of deep learning impact on AES’s predictive and descriptive accuracies.

Table 1. Hyperparameter subspace investigated in this article along with best hyperparameter values per neural network architecture.

Consequently, the 1567 labeled essays were divided into a training set (80%) and a testing set (20%). No validation set was put aside; 5-fold cross-validation was rather used for hyperparameter optimization. Table 1 delineates the hyperparameter subspace from which 800 different combinations of hyperparameter values were randomly selected out of a subspace of 86,248,800 possible combinations. Since this research proposes to investigate the potential of deep learning to predict rubric scores, several architectures consisting of 2 to 6 hidden layers and ranging from 9,156 to 119,312 parameters were tested. Table 1 shows the best hyperparameter values per depth of neural networks.

Again, the essays of the testing set were never used during the training and cross-validation processes. In order to retrieve the best predictive models during training, every time the validation loss reached a record low, the model was overwritten. Training stopped when no new record low was reached during 100 epochs. Moreover, to avoid reporting the performance of overfit models, each model was trained five times using the same set of best hyperparameter values. Finally, for each resulting predictive model, a corresponding ensemble model (bagging) was also obtained out of the five models trained during cross-validation.

Predictive Models and Predictive Accuracy

Table 2 delineates the performance of predictive models trained previously by Kumar and Boulanger (2020) on the four scoring rubrics. The first row lists the agreement levels between the resolved and predicted rubric scores measured by the quadratic weighted kappa. The second row is the percentage of accurate predictions; the third row reports the percentages of predictions that are either accurate or off by 1; and the fourth row reports the percentages of predictions that are either accurate or at most off by 2. Prediction of holistic scores is done merely by adding up all rubric scores. Since the scale of rubric scores is 0−6 for every rubric, then the scale of holistic scores is 0−24.

Table 2. Rubric scoring models’ performance on testing set.

While each of these rubric scoring models might suffer from its own systemic bias and hence cancel off each other’s bias by adding up the rubric scores to derive the holistic score, this study (unlike related works) intends to highlight these biases by exposing the decision making process underlying the prediction of rubric scores. Although this paper exclusively focuses on the Style rubric, the methodology put forward to analyze the local and global importance of writing indices and their context-specific contributions to predicted rubric scores is applicable to every rubric and allows to control for these biases one rubric at a time. Comparing and contrasting the role that a specific writing index plays within each rubric context deserves its own investigation, which has been partly addressed in the study led by Kumar and Boulanger (2020) . Moreover, this paper underscores the necessity to measure the predictive accuracy of rubric-based holistic scoring using additional metrics to account for these rubric-specific biases. For example, there exist several combinations of rubric scores to obtain a holistic score of 16 (e.g., 4-4-4-4 vs. 4-3-4-5 vs. 3-5-2-6). Even though the predicted holistic score might be accurate, the rubric scores could all be inaccurate. Similarity or distance metrics (e.g., Manhattan and Euclidean) should then be used to describe the authenticity of the composition of these holistic scores.

According to what Kumar and Boulanger (2020) report on the performance of several state-of-the-art AES systems trained on ASAP’s seventh essay dataset, the AES system they developed and which will be reused in this paper proved competitive while being fully and deeply interpretable, which no other AES system does. They also supply further information about the study setting, essay datasets, rubrics, features, natural language processing (NLP) tools, model training, and evaluation against human performance. Again, this paper showcases the application of explainable artificial intelligence in automated essay scoring by focusing on the decision process of the Rubric #3 (Style) scoring model. Remember that the same methodology is applicable to each rubric.

Explanation Model: SHAP

SH apley A dditive ex P lanations (SHAP) is a theoretically justified XAI framework that can provide simultaneously both local and global explanations ( Molnar, 2020 ); that is, SHAP is able to explain individual predictions taking into account the uniqueness of each prediction, while highlighting the global factors influencing the overall performance of a predictive model. SHAP is of keen interest because it unifies all algorithms of the class of additive feature attribution methods, adhering to a set of three properties that are desirable in interpretable machine learning: local accuracy, missingness, and consistency ( Lundberg and Lee, 2017 ). A key advantage of SHAP is that feature contributions are all expressed in terms of the outcome variable (e.g., rubric scores), providing a same scale to compare the importance of each feature against each other. Local accuracy refers to the fact that no matter the explanation model, the sum of all feature contributions is always equal to the prediction explained by these features. The missingness property implies that the prediction is never explained by unmeasured factors, which are always assigned a contribution of zero. However, the converse is not true; a contribution of zero does not imply an unobserved factor, it can also denote a feature irrelevant to explain the prediction. The consistency property guarantees that a more important feature will always have a greater magnitude than a less important one, no matter how many other features are included in the explanation model. SHAP proves superior to other additive attribution methods such as LIME (Local Interpretable Model-Agnostic Explanations), Shapley values, and DeepLIFT in that they never comply with all three properties, while SHAP does ( Lundberg and Lee, 2017 ). Moreover, the way SHAP assesses the importance of a feature differs from permutation importance methods (e.g., ELI5), measured as the decrease in model performance (accuracy) as a feature is perturbated, in that it is based on how much a feature contributes to every prediction.

Essentially, a SHAP explanation model (linear regression) is trained on top of a predictive model, which in this case is a complex ensemble deep learning model. Table 3 demonstrates a scale explanation model showing how SHAP values (feature contributions) work. In this example, there are five instances and five features describing each instance (in the context of this paper, an instance is an essay). Predictions are listed in the second to last column, and the base value is the mean of all predictions. The base value constitutes the reference point according to which predictions are explained; in other words, reasons are given to justify the discrepancy between the individual prediction and the mean prediction (the base value). Notice that the table does not contain the actual feature values; these are SHAP values that quantify the contribution of each feature to the predicted score. For example, the prediction of Instance 1 is 2.46, while the base value is 3.76. Adding up the feature contributions of Instance 1 to the base value produces the predicted score:

Table 3. Array of SHAP values: local and global importance of features and feature coverage per instance.

Hence, the generic equation of the explanation model ( Lundberg and Lee, 2017 ) is:

where g(x) is the prediction of an individual instance x, σ 0 is the base value, σ i is the feature contribution of feature x i , x i ∈ {0,1} denotes whether feature x i is part of the individual explanation, and j is the total number of features. Furthermore, the global importance of a feature is calculated by adding up the absolute values of its corresponding SHAP values over all instances, where n is the total number of instances and σ i ( j ) is the feature contribution for instance i ( Lundberg et al., 2018 ):

Therefore, it can be seen that Feature 3 is the most globally important feature, while Feature 2 is the least important one. Similarly, Feature 5 is Instance 3’s most important feature at the local level, while Feature 2 is the least locally important. The reader should also note that a feature shall not necessarily be assigned any contribution; some of them are just not part of the explanation such as Feature 2 and Feature 3 in Instance 2. These concepts lay the foundation for the explainable AES system presented in this paper. Just imagine that each instance (essay) will be rather summarized by 282 features and that the explanations of all the testing set’s 314 essays will be provided.

Several implementations of SHAP exist: KernelSHAP, DeepSHAP, GradientSHAP, and TreeSHAP, among others. KernelSHAP is model-agnostic and works for any type of predictive models; however, KernelSHAP is very computing-intensive which makes it undesirable for practical purposes. DeepSHAP and GradientSHAP are two implementations intended for deep learning which takes advantage of the known properties of neural networks (i.e., MLP-NN, CNN, or RNN) to accelerate up to three orders of magnitude the processing time to explain predictions ( Chen et al., 2019 ). Finally, TreeSHAP is the most powerful implementation intended for tree-based models. TreeSHAP is not only fast; it is also accurate. While the three former implementations estimate SHAP values, TreeSHAP computes them exactly. Moreover, TreeSHAP not only measures the contribution of individual features, but it also considers interactions between pairs of features and assigns them SHAP values. Since one of the goals of this paper is to assess the potential of deep learning on the performance of both predictive and explanation models, this research tested the former three implementations. TreeSHAP is recommended for future work since the interaction among features is critical information to consider. Moreover, KernelSHAP, DeepSHAP, and GradientSHAP all require access to the whole original dataset to derive the explanation of a new instance, another constraint TreeSHAP is not subject to.

Descriptive Accuracy: Trustworthiness of Explanation Models

This paper reuses and adapts the methodology introduced by Ribeiro et al. (2016) . Several explanation models will be trained, using different SHAP implementations and configurations, per deep learning predictive model (for each number of hidden layers). The rationale consists in randomly selecting and ignoring 25% of the 282 features feeding the predictive model (e.g., turning them to zero). If it causes the prediction to change beyond a specific threshold (in this study 0.10 and 0.25 were tested), then the explanation model should also reflect the magnitude of this change while ignoring the contributions of these same features. For example, the original predicted rubric score of an essay might be 5; however, when ignoring the information brought in by a subset of 70 randomly selected features (25% of 282), the prediction may turn to 4. On the other side, if the explanation model also predicts a 4 while ignoring the contributions of the same subset of features, then the explanation is considered as trustworthy. This allows to compute the precision, recall, and F1-score of each explanation model (number of true and false positives and true and false negatives). The process is repeated 500 times for every essay to determine the average precision and recall of every explanation model.

Judging Relevancy

So far, the consistency of explanations with predictions has been considered. However, consistent explanations do not imply relevant or meaningful explanations. Put another way, explanations only reflect what predictive models have learned during training. How can the black box of these explanations be opened? Looking directly at the numerical SHAP values of each explanation might seem a daunting task, but there exist tools, mainly visualizations (decision plot, summary plot, and dependence plot), that allow to make sense out of these explanations. However, before visualizing these explanations, another question needs to be addressed: which explanations or essays should be picked for further scrutiny of the AES system? Given the huge number of essays to examine and the tedious task to understand the underpinnings of a single explanation, a small subset of essays should be carefully picked that should represent concisely the state of correctness of the underlying predictive model. Again, this study applies and adapts the methodology in Ribeiro et al. (2016) . A greedy algorithm selects essays whose predictions are explained by as many features of global importance as possible to optimize feature coverage. Ribeiro et al. demonstrated in unrelated studies (i.e., sentiment analysis) that the correctness of a predictive model can be assessed with as few as four or five well-picked explanations.

For example, Table 3 reveals the global importance of five features. The square root of each feature’s global importance is also computed and considered instead to limit the influence of a small group of very influential features. The feature coverage of Instance 1 is 100% because all features are engaged in the explanation of the prediction. On the other hand, Instance 2 has a feature coverage of 61.5% because only Features 1, 4, and 5 are part of the prediction’s explanation. The feature coverage is calculated by summing the square root of each explanation’s feature’s global importance together and dividing by the sum of the square roots of all features’ global importance:

Additionally, it can be seen that Instance 4 does not have any zero-feature value although its feature coverage is only 84.6%. The algorithm was constrained to discard from the explanation any feature whose contribution (local importance) was too close to zero. In the case of Table 3 ’s example, any feature whose absolute SHAP value is less than 0.10 is ignored, hence leading to a feature coverage of:

In this paper’s study, the real threshold was 0.01. This constraint was actually a requirement for the DeepSHAP and GradientSHAP implementations because they only output non-zero SHAP values contrary to KernelSHAP which generates explanations with a fixed number of features: a non-zero SHAP value indicates that the feature is part of the explanation, while a zero value excludes the feature from the explanation. Without this parameter, all 282 features would be part of the explanation although a huge number only has a trivial (very close to zero) SHAP value. Now, a much smaller but variable subset of features makes up each explanation. This is one way in which Ribeiro et al.’s SP-LIME algorithm (SP stands for Submodular Pick) has been adapted to this study’s needs. In conclusion, notice how Instance 4 would be selected in preference to Instance 5 to explain Table 3 ’s underlying predictive model. Even though both instances have four features explaining their prediction, Instance 4’s features are more globally important than Instance 5’s features, and therefore Instance 4 has greater feature coverage than Instance 5.

Whereas Table 3 ’s example exhibits the feature coverage of one instance at a time, this study computes it for a subset of instances, where the absolute SHAP values are aggregated (summed) per candidate subset. When the sum of absolute SHAP values per feature exceeds the set threshold, the feature is then considered as covered by the selected set of instances. The objective in this study was to optimize the feature coverage while minimizing the number of essays to validate the AES model.

Research Questions

One of this article’s objectives is to assess the potential of deep learning in automated essay scoring. The literature has often claimed ( Hussein et al., 2019 ) that there are two approaches to AES, feature-based and deep learning, as though these two approaches were mutually exclusive. Yet, the literature also puts forward that feature-based AES models may be more interpretable than deep learning ones ( Amorim et al., 2018 ). This paper embraces the viewpoint that these two approaches can also be complementary by leveraging the state-of-the-art in NLP and automatic linguistic analysis and harnessing one of the richest pools of linguistic indices put forward in the research community ( Crossley et al., 2016 , 2017 , 2019 ; Kyle, 2016 ; Kyle et al., 2018 ) and applying a thorough feature selection process powered by deep learning. Moreover, the ability of deep learning of modeling complex non-linear relationships makes it particularly well-suited for AES given that the importance of a writing feature is highly dependent on its context, that is, its interactions with other writing features. Besides, this study leverages the SHAP interpretation method that is well-suited to interpret very complex models. Hence, this study elected to work with deep learning models and ensembles to test SHAP’s ability to explain these complex models. Previously, the literature has revealed the difficulty to have at the same time both accurate and interpretable models ( Ribeiro et al., 2016 ; Murdoch et al., 2019 ), where favoring one comes at the expense of the other. However, this research shows how XAI makes it now possible to produce both accurate and interpretable models in the area of AES. Since ensembles have been repeatedly shown to boost the accuracy of predictive models, they were included as part of the tested deep learning architectures to maximize generalizability and accuracy, while making these predictive models interpretable and exploring whether deep learning can even enhance their descriptive accuracy further.

This study investigates the trustworthiness of explanation models, and more specifically, those explaining deep learning predictive models. For instance, does the depth, defined as the number of hidden layers, of an MLP neural network increases the trustworthiness of its SHAP explanation model? The answer to this question will help determine whether it is possible to have very accurate AES models while having competitively interpretable/explainable models, the corner stone for the generation of formative feedback. Remember that formative feedback is defined as “any kind of information provided to students about their actual state of learning or performance in order to modify the learner’s thinking or behavior in the direction of the learning standards” and that formative feedback “conveys where the student is, what are the goals to reach, and how to reach the goals” ( Goldin et al., 2017 ). This notion contrasts with summative feedback which basically is “a justification of the assessment results” ( Hao and Tsikerdekis, 2019 ).

As pointed out in the previous section, multiple SHAP implementations are evaluated in this study. Hence, this paper showcases whether the faster DeepSHAP and GradientSHAP implementations are as reliable as the slower KernelSHAP implementation . The answer to this research question will shed light on the feasibility of providing immediate formative feedback and this multiple times throughout students’ writing processes.

This study also looks at whether a summary of the data produces as trustworthy explanations as those from the original data . This question will be of interest to AES researchers and practitioners because it could allow to significantly decrease the processing time of the computing-intensive and model-agnostic KernelSHAP implementation and test further the potential of customizable explanations.

KernelSHAP allows to specify the total number of features that will shape the explanation of a prediction; for instance, this study experiments with explanations of 16 and 32 features and observes whether there exists a statistically significant difference in the reliability of these explanation models . Knowing this will hint at whether simpler or more complex explanations are more desirable when it comes to optimize their trustworthiness. If there is no statistically significant difference, then AES practitioners are given further flexibility in the selection of SHAP implementations to find the sweet spot between complexity of explanations and speed of processing. For instance, the KernelSHAP implementation allows to customize the number of factors making up an explanation, while the faster DeepSHAP and GradientSHAP do not.

Finally, this paper highlights the means to debug and compare the performance of predictive models through their explanations. Once a model is debugged, the process can be reused to fine-tune feature selection and/or feature engineering to improve predictive models and for the generation of formative feedback to both students and teachers.

The training, validation, and testing sets consist of 1567 essays, each of which has been scored by two human raters, who assigned a score between 0 and 3 per rubric (ideas, organization, style, and conventions). In particular, this article looks at predictive and descriptive accuracy of AES models on the third rubric, style. Note that although each essay has been scored by two human raters, the literature ( Shermis, 2014 ) is not explicit about whether only two or more human raters participated in the scoring of all 1567 essays; given the huge number of essays, it is likely that more than two human raters were involved in the scoring of these essays so that the amount of noise introduced by the various raters’ biases is unknown while probably being at some degree balanced among the two groups of raters. Figure 2 shows the confusion matrices of human raters on Style Rubric. The diagonal elements (dark gray) correspond to exact matches, whereas the light gray squares indicate adjacent matches. Figure 2A delineates the number of essays per pair of ratings, and Figure 2B shows the percentages per pair of ratings. The agreement level between each pair of human raters, measured by the quadratic weighted kappa, is 0.54; the percentage of exact matches is 65.3%; the percentage of adjacent matches is 34.4%; and 0.3% of essays are neither exact nor adjacent matches. Figures 2A,B specify the distributions of 0−3 ratings per group of human raters. Figure 2C exhibits the distribution of resolved scores (a resolved score is the sum of the two human ratings). The mean is 3.99 (with a standard deviation of 1.10), and the median and mode are 4. It is important to note that the levels of predictive accuracy reported in this article are measured on the scale of resolved scores (0−6) and that larger scales tend to slightly inflate quadratic weighted kappa values, which must be taken into account when comparing against the level of agreement between human raters. Comparison of percentages of exact and adjacent matches must also be made with this scoring scale discrepancy in mind.

Figure 2. Summary of the essay dataset (1567 Grade-7 narrative essays) investigated in this study. (A) Number of essays per pair of human ratings; the diagonal (dark gray squares) lists the numbers of exact matches while the light-gray squares list the numbers of adjacent matches; and the bottom row and the rightmost column highlight the distributions of ratings for both groups of human raters. (B) Percentages of essays per pair of human ratings; the diagonal (dark gray squares) lists the percentages of exact matches while the light-gray squares list the percentages of adjacent matches; and the bottom row and the rightmost column highlight the distributions (frequencies) of ratings for both groups of human raters. (C) The distribution of resolved rubric scores; a resolved score is the addition of its two constituent human ratings.

Predictive Accuracy and Descriptive Accuracy

Table 4 compiles the performance outcomes of the 10 predictive models evaluated in this study. The reader should remember that the performance of each model was averaged over five iterations and that two models were trained per number of hidden layers, one non-ensemble and one ensemble. Except for the 6-layer models, there is no clear winner among other models. Even for the 6-layer models, they are superior in terms of exact matches, the primary goal for a reliable AES system, but not according to adjacent matches. Nevertheless, on average ensemble models slightly outperform non-ensemble models. Hence, these ensemble models will be retained for the next analysis step. Moreover, given that five ensemble models were trained per neural network depth, the most accurate model among the five is selected and displayed in Table 4 .

Table 4. Performance of majority classifier and average/maximal performance of trained predictive models.

Next, for each selected ensemble predictive model, several explanation models are trained per predictive model. Every predictive model is explained by the “Deep,” “Grad,” and “Random” explainers, except for the 6-layer model where it was not possible to train a “Deep” explainer apparently due to a bug in the original SHAP code caused by either a unique condition in this study’s data or neural network architecture. However, this was beyond the scope of this study to fix and investigate this issue. As it will be demonstrated, no statistically significant difference exists between the accuracy of these explainers.

The “Random” explainer serves as a baseline model for comparison purpose. Remember that to evaluate the reliability of explanation models, the concurrent impact of randomly selecting and ignoring a subset of features on the prediction and explanation of rubric scores is analyzed. If the prediction changes significantly and its corresponding explanation changes (beyond a set threshold) accordingly (a true positive) or if the prediction remains within the threshold as does the explanation (a true negative), then the explanation is deemed as trustworthy. Hence, in the case of the Random explainer, it simulates random explanations by randomly selecting 32 non-zero features from the original set of 282 features. These random explanations consist only of non-zero features because, according to SHAP’s missingness property, a feature with a zero or a missing value never gets assigned any contribution to the prediction. If at least one of these 32 features is also an element of the subset of the ignored features, then the explanation is considered as untrustworthy, no matter the size of a feature’s contribution.

As for the layer-2 model, six different explanation models are evaluated. Recall that layer-2 models generated the least mean squared error (MSE) during hyperparameter optimization (see Table 1 ). Hence, this specific type of architecture was selected to test the reliability of these various explainers. The “Kernel” explainer is the most computing-intensive and took approximately 8 h of processing. It was trained using the full distributions of feature values in the training set and shaped explanations in terms of 32 features; the “Kernel-16” and “Kernel-32” models were trained on a summary (50 k -means centroids) of the training set to accelerate the processing by about one order of magnitude (less than 1 h). Besides, the “Kernel-16” explainer derived explanations in terms of 16 features, while the “Kernel-32” explainer explained predictions through 32 features. Table 5 exhibits the descriptive accuracy of these various explanation models according to a 0.10 and 0.25 threshold; in other words, by ignoring a subset of randomly picked features, it assesses whether or not the prediction and explanation change simultaneously. Note also how each explanation model, no matter the underlying predictive model, outperforms the “Random” model.

Table 5. Precision, recall, and F1 scores of the various explainers tested per type of predictive model.

The first research question addressed in this subsection asks whether there exists a statistically significant difference between the “Kernel” explainer, which generates 32-feature explanations and is trained on the whole training set, and the “Kernel-32” explainer which also generates 32-feature explanations and is trained on a summary of the training set. To determine this, an independent t-test was conducted using the precision, recall, and F1-score distributions (500 iterations) of both explainers. Table 6 reports the p -values of all the tests and for the 0.10 and 0.25 thresholds. It reveals that there is no statistically significant difference between the two explainers.

Table 6. p -values of independent t -tests comparing whether there exist statistically significant differences between the mean precisions, recalls, and F1-scores of 2-layer explainers and between those of the 2-layer’s, 4-layer’s, and 6-layer’s Gradient explainers.

The next research question tests whether there exists a difference in the trustworthiness of explainers shaping 16 or 32-feature explanations. Again t-tests were conducted to verify this. Table 6 lists the resulting p -values. Again, there is no statistically significant difference in the average precisions, recalls, and F1-scores of both explainers.

This leads to investigating whether the “Kernel,” “Deep,” and “Grad” explainers are equivalent. Table 6 exhibits the results of the t-tests conducted to verify this and reveals that none of the explainers produce a statistically significantly better performance than the other.

Armed with this evidence, it is now possible to verify whether deeper MLP neural networks produce more trustworthy explanation models. For this purpose, the performance of the “Grad” explainer for each type of predictive model will be compared against each other. The same methodology as previously applied is employed here. Table 6 , again, confirms that the explanation model of the 2-layer predictive model is statistically significantly less trustworthy than the 4-layer’s explanation model; the same can be said of the 4-layer and 6-layer models. The only exception is the difference in average precision between 2-layer and 4-layer models and between 4-layer and 6-layer models; however, there clearly exists a statistically significant difference in terms of precision (and also recall and F1-score) between 2-layer and 6-layer models.

The Best Subset of Essays to Judge AES Relevancy

Table 7 lists the four best essays optimizing feature coverage (93.9%) along with their resolved and predicted scores. Notice how two of the four essays were picked by the adapted SP-LIME algorithm with some strong disagreement between the human and the machine graders, two were picked with short and trivial text, and two were picked exhibiting perfect agreement between the human and machine graders. Interestingly, each pair of longer and shorter essays exposes both strong agreement and strong disagreement between the human and AI agents, offering an opportunity to debug the model and evaluate its ability to detect the presence or absence of more basic (e.g., very small number of words, occurrences of sentence fragments) and more advanced aspects (e.g., cohesion between adjacent sentences, variety of sentence structures) of narrative essay writing and to appropriately reward or penalize them.

Table 7. Set of best essays to evaluate the correctness of the 6-layer ensemble AES model.

Local Explanation: The Decision Plot

The decision plot lists writing features by order of importance from top to bottom. The line segments display the contribution (SHAP value) of each feature to the predicted rubric score. Note that an actual decision plot consists of all 282 features and that only the top portion of it (20 most important features) can be displayed (see Figure 3 ). A decision plot is read from bottom to top. The line starts at the base value and ends at the predicted rubric score. Given that the “Grad” explainer is the only explainer common to all predictive models, it has been selected to derive all explanations. The decision plots in Figure 3 show the explanations of the four essays in Table 7 ; the dashed line in these plots represents the explanation of the most accurate predictive model, that is the ensemble model with 6 hidden layers which also produced the most trustworthy explanation model. The predicted rubric score of each explanation model is listed in the bottom-right legend. Explanation of the writing features follow in a next subsection.

Figure 3. Comparisons of all models’ explanations of the most representative set of four essays: (A) Essay 228, (B) Essay 68, (C) Essay 219, and (D) Essay 124.

Global Explanation: The Summary Plot

It is advantageous to use SHAP to build explanation models because it provides a single framework to discover the writing features that are important to an individual essay (local) or a set of essays (global). While the decision plots list features of local importance, Figure 4 ’s summary plot ranks writing features by order of global importance (from top to bottom). All testing set’s 314 essays are represented as dots in the scatterplot of each writing feature. The position of a dot on the horizontal axis corresponds to the importance (SHAP value) of the writing feature for a specific essay and its color indicates the magnitude of the feature value in relation to the range of all 314 feature values. For example, large or small numbers of words within an essay generally contribute to increase or decrease rubric scores by up to 1.5 and 1.0, respectively. Decision plots can also be used to find the most important features for a small subset of essays; Figure 5 demonstrates the new ordering of writing indices when aggregating the feature contributions (summing the absolute values of SHAP values) of the four essays in Table 7 . Moreover, Figure 5 allows to compare the contributions of a feature to various essays. Note how the orderings in Figures 3 −5 can differ from each other, sharing many features of global importance as well as having their own unique features of local importance.

Figure 4. Summary plot listing the 32 most important features globally.

Figure 5. Decision plot delineating the best model’s explanations of Essays 228, 68, 219, and 124 (6-layer ensemble).

Definition of Important Writing Indices

The reader shall understand that it is beyond the scope of this paper to make a thorough description of all writing features. Nevertheless, the summary and decision plots in Figures 4 , 5 allow to identify a subset of features that should be examined in order to validate this study’s predictive model. Supplementary Table 1 combines and describes the 38 features in Figures 4 , 5 .

Dependence Plots

Although the summary plot in Figure 4 is insightful to determine whether small or large feature values are desirable, the dependence plots in Figure 6 prove essential to recommend whether a student should aim at increasing or decreasing the value of a specific writing feature. The dependence plots also reveal whether the student should directly act upon the targeted writing feature or indirectly on other features. The horizontal axis in each of the dependence plots in Figure 6 is the scale of the writing feature and the vertical axis is the scale of the writing feature’s contributions to the predicted rubric scores. Each dot in a dependence plot represents one of the testing set’s 314 essays, that is, the feature value and SHAP value belonging to the essay. The vertical dispersion of the dots on small intervals of the horizontal axis is indicative of interaction with other features ( Molnar, 2020 ). If the vertical dispersion is widespread (e.g., the [50, 100] horizontal-axis interval in the “word_count” dependence plot), then the contribution of the writing feature is most likely at some degree dependent on other writing feature(s).

Figure 6. Dependence plots: the horizontal axes represent feature values while vertical axes represent feature contributions (SHAP values). Each dot represents one of the 314 essays of the testing set and is colored according to the value of the feature with which it interacts most strongly. (A) word_count. (B) hdd42_aw. (C) ncomp_stdev. (D) dobj_per_cl. (E) grammar. (F) SENTENCE_FRAGMENT. (G) Sv_GI. (H) adjacent_overlap_verb_sent.

The contributions of this paper can be summarized as follows: (1) it proposes a means (SHAP) to explain individual predictions of AES systems and provides flexible guidelines to build powerful predictive models using more complex algorithms such as ensembles and deep learning neural networks; (2) it applies a methodology to quantitatively assess the trustworthiness of explanation models; (3) it tests whether faster SHAP implementations impact the descriptive accuracy of explanation models, giving insight on the applicability of SHAP in real pedagogical contexts such as AES; (4) it offers a toolkit to debug AES models, highlights linguistic intricacies, and underscores the means to offer formative feedback to novice writers; and more importantly, (5) it empowers learning analytics practitioners to make AI pedagogical agents accountable to the human educator, the ultimate problem holder responsible for the decisions and actions of AI ( Abbass, 2019 ). Basically, learning analytics (which encompasses tools such as AES) is characterized as an ethics-bound, semi-autonomous, and trust-enabled human-AI fusion that recurrently measures and proactively advances knowledge boundaries in human learning.

To exemplify this, imagine an AES system that supports instructors in the detection of plagiarism, gaming behaviors, and the marking of writing activities. As previously mentioned, essays are marked according to a grid of scoring rubrics: ideas, organization, style, and conventions. While an abundance of data (e.g., the 1592 writing metrics) can be collected by the AES tool, these data might still be insufficient to automate the scoring process of certain rubrics (e.g., ideas). Nevertheless, some scoring subtasks such as assessing a student’s vocabulary, sentence fluency, and conventions might still be assigned to AI since the data types available through existing automatic linguistic analysis tools prove sufficient to reliably alleviate the human marker’s workload. Interestingly, learning analytics is key for the accountability of AI agents to the human problem holder. As the volume of writing data (through a large student population, high-frequency capture of learning episodes, and variety of big learning data) accumulate in the system, new AI agents (predictive models) may apply for the job of “automarker.” These AI agents can be quite transparent through XAI ( Arrieta et al., 2020 ) explanation models, and a human instructor may assess the suitability of an agent for the job and hire the candidate agent that comes closest to human performance. Explanations derived from these models could serve as formative feedback to the students.

The AI marker can be assigned to assess the writing activities that are similar to those previously scored by the human marker(s) from whom it learns. Dissimilar and unseen essays can be automatically assigned to the human marker for reliable scoring, and the AI agent can learn from this manual scoring. To ensure accountability, students should be allowed to appeal the AI agent’s marking to the human marker. In addition, the human marker should be empowered to monitor and validate the scoring of select writing rubrics scored by the AI marker. If the human marker does not agree with the machine scores, the writing assignments may be flagged as incorrectly scored and re-assigned to a human marker. These flagged assignments may serve to update predictive models. Moreover, among the essays that are assigned to the machine marker, a small subset can be simultaneously assigned to the human marker for continuous quality control; that is, to continue comparing whether the agreement level between human and machine markers remains within an acceptable threshold. The human marker should be at any time able to “fire” an AI marker or “hire” an AI marker from a pool of potential machine markers.

This notion of a human-AI fusion has been observed in previous AES systems where the human marker’s workload has been found to be significantly alleviated, passing from scoring several hundreds of essays to just a few dozen ( Dronen et al., 2015 ; Hellman et al., 2019 ). As the AES technology matures and as the learning analytics tools continue to penetrate the education market, this alliance of semi-autonomous human and AI agents will lead to better evidence-based/informed pedagogy ( Nelson and Campbell, 2017 ). Such a human-AI alliance can also be guided to autonomously self-regulate its own hypothesis-authoring and data-acquisition processes for purposes of measuring and advancing knowledge boundaries in human learning.

Real-Time Formative Pedagogical Feedback

This paper provides the evidence that deep learning and SHAP can be used not only to score essays automatically but also to offer explanations in real-time. More specifically, the processing time to derive the 314 explanations of the testing set’s essays has been benchmarked for several types of explainers. It was found that the faster DeepSHAP and GradientSHAP implementations, which took only a few seconds of processing, did not produce less accurate explanations than the much slower KernelSHAP. KernelSHAP took approximately 8 h of processing to derive the explanation model of a 2-layer MLP neural network predictive model and 16 h for the 6-layer predictive model.

This finding also holds for various configurations of KernelSHAP, where the number of features (16 vs. 32) shaping the explanation (where all other features are assigned zero contributions) did not produce a statistically significant difference in the reliability of the explanation models. On average, the models had a precision between 63.9 and 64.1% and a recall between 41.0 and 42.9%. This means that after perturbation of the predictive and explanation models, on average 64% of the predictions the explanation model identified as changing were accurate. On the other side, only about 42% of all predictions that changed were detected by the various 2-layer explainers. An explanation was considered as untrustworthy if the sum of its feature contributions, when added to the average prediction (base value), was not within 0.1 from the perturbated prediction. Similarly, the average precision and recall of 2-layer explainers for the 0.25-threshold were about 69% and 62%, respectively.

Impact of Deep Learning on Descriptive Accuracy of Explanations

By analyzing the performance of the various predictive models in Table 4 , no clear conclusion can be reached as to which model should be deemed as the most desirable. Despite the fact that the 6-layer models slightly outperform the other models in terms of accuracy (percentage of exact matches between the resolved [human] and predicted [machine] scores), they are not the best when it comes to the percentages of adjacent (within 1 and 2) matches. Nevertheless, if the selection of the “best” model is based on the quadratic weighted kappas, the decision remains a nebulous one to make. Moreover, ensuring that machine learning actually learned something meaningful remains paramount, especially in contexts where the performance of a majority classifier is close to the human and machine performance. For example, a majority classifier model would get 46.3% of predictions accurate ( Table 4 ), while trained predictive models at best produce accurate predictions between 51.9 and 55.1%.

Since the interpretability of a machine learning model should be prioritized over accuracy ( Ribeiro et al., 2016 ; Murdoch et al., 2019 ) for questions of transparency and trust, this paper investigated whether the impact of the depth of a MLP neural network might be more visible when assessing its interpretability, that is, the trustworthiness of its corresponding SHAP explanation model. The data in Tables 1 , 5 , 6 effectively support the hypothesis that as the depth of the neural network increases, the precision and recall of the corresponding explanation model improve. Besides, this observation is particularly interesting because the 4-layer (Grad) explainer, which has hardly more parameters than the 2-layer model, is also more accurate than the 2-layer model, suggesting that the 6-layer explainer is most likely superior to other explainers not only because of its greater number of parameters, but also because of its number of hidden layers. By increasing the number of hidden layers, it can be seen that the precision and recall of an explanation model can pass on average from approximately 64 to 73% and from 42 to 52%, respectively, for the 0.10-threshold; and for the 0.25-threshold, from 69 to 79% and from 62 to 75%, respectively.

These results imply that the descriptive accuracy of an explanation model is an evidence of effective machine learning, which may exceed the level of agreement between the human and machine graders. Moreover, given that the superiority of a trained predictive model over a majority classifier is not always obvious, the consistency of its associated explanation model demonstrates this better. Note that theoretically the SHAP explanation model of the majority classifier should assign a zero contribution to each writing feature since the average prediction of such a model is actually the most frequent rubric score given by the human raters; hence, the base value is the explanation.

An interesting fact emerges from Figure 3 , that is, all explainers (2-layer to 6-layer) are more or less similar. It appears that they do not contradict each other. More specifically, they all agree on the direction of the contributions of the most important features. In other words, they unanimously determine that a feature should increase or decrease the predicted score. However, they differ from each other on the magnitude of the feature contributions.

To conclude, this study highlights the need to train predictive models that consider the descriptive accuracy of explanations. The idea is that explanation models consider predictions to derive explanations; explanations should be considered when training predictive models. This would not only help train interpretable models the very first time but also potentially break the status quo that may exist among similar explainers to possibly produce more powerful models. In addition, this research calls for a mechanism (e.g., causal diagrams) to allow teachers to guide the training process of predictive models. Put another way, as LA practitioners debug predictive models, their insights should be encoded in a language that will be understood by the machine and that will guide the training process to avoid learning the same errors and to accelerate the training time.

Accountable AES

Now that the superiority of the 6-layer predictive and explanation models has been demonstrated, some aspects of the relevancy of explanations should be examined more deeply, knowing that having an explanation model consistent with its underlying predictive model does not guarantee relevant explanations. Table 7 discloses the set of four essays that optimize the coverage of most globally important features to evaluate the correctness of the best AES model. It is quite intriguing to note that two of the four essays are among the 16 essays that have a major disagreement (off by 2) between the resolved and predicted rubric scores (1 vs. 3 and 4 vs. 2). The AES tool clearly overrated Essay 228, while it underrated Essay 219. Naturally, these two essays offer an opportunity to understand what is wrong with the model and ultimately debug the model to improve its accuracy and interpretability.

In particular, Essay 228 raises suspicion on the positive contributions of features such as “Ortho_N,” “lemma_mattr,” “all_logical,” “det_pobj_deps_struct,” and “dobj_per_cl.” Moreover, notice how the remaining 262 less important features (not visible in the decision plot in Figure 5 ) have already inflated the rubric score beyond the base value, more than any other essay. Given the very short length and very low quality of the essay, whose meaning is seriously undermined by spelling and grammatical errors, it is of utmost importance to verify how some of these features are computed. For example, is the average number of orthographic neighbors (Ortho_N) per token computed for unmeaningful tokens such as “R” and “whe”? Similarly, are these tokens considered as types in the type-token ratio over lemmas (lemma_mattr)? Given the absence of a meaningful grammatical structure conveying a complete idea through well-articulated words, it becomes obvious that the quality of NLP (natural language processing) parsing may become a source of (measurement) bias impacting both the way some writing features are computed and the predicted rubric score. To remedy this, two solutions are proposed: (1) enhancing the dataset with the part-of-speech sequence or the structure of dependency relationships along with associated confidence levels, or (2) augmenting the essay dataset with essays enclosing various types of non-sensical content to improve the learning of these feature contributions.

Note that all four essays have a text length smaller than the average: 171 words. Notice also how the “hdd42_aw” and “hdd42_fw” play a significant role to decrease the predicted score of Essays 228 and 68. The reader should note that these metrics require a minimum of 42 tokens in order to compute a non-zero D index, a measure of lexical diversity as explained in Supplementary Table 1 . Figure 6B also shows how zero “hdd42_aw” values are heavily penalized. This is extra evidence that supports the strong role that the number of words plays in determining these rubric scores, especially for very short essays where it is one of the few observations that can be reliably recorded.

Two other issues with the best trained AES model were identified. First, in the eyes of the model, the lowest the average number of direct objects per clause (dobj_per_cl), as seen in Figure 6D , the best it is. This appears to contradict one of the requirements of the “Style” rubric, which looks for a variety of sentence structures. Remember that direct objects imply the presence of transitive verbs (action verbs) and that the balanced usage of linking verbs and action verbs as well as of transitive and intransitive verbs is key to meet the requirement of variety of sentence structures. Moreover, note that the writing feature is about counting the number of direct objects per clause, not by sentence. Only one direct object is therefore possible per clause. On the other side, a sentence may contain several clauses, which determines if the sentence is a simple, compound, or a complex sentence. This also means that a sentence may have multiple direct objects and that a high ratio of direct objects per clause is indicative of sentence complexity. Too much complexity is also undesirable. Hence, it is fair to conclude that the higher range of feature values has reasonable feature contributions (SHAP values), while the lower range does not capture well the requirements of the rubric. The dependence plot should rather display a positive peak somewhere in the middle. Notice how the poor quality of Essay 228’s single sentence prevented the proper detection of the single direct object, “broke my finger,” and the so-called absence of direct objects was one of the reasons to wrongfully improve the predicted rubric score.

The model’s second issue discussed here is the presence of sentence fragments, a type of grammatical errors. Essentially, a sentence fragment is a clause that misses one of three critical components: a subject, a verb, or a complete idea. Figure 6E shows the contribution model of grammatical errors, all types combined, while Figure 6F shows specifically the contribution model of sentence fragments. It is interesting to see how SHAP further penalizes larger numbers of grammatical errors and that it takes into account the length of the essay (red dots represent essays with larger numbers of words; blue dots represent essays with smaller numbers of words). For example, except for essays with no identified grammatical errors, longer essays are less penalized than shorter ones. This is particularly obvious when there are 2−4 grammatical errors. The model increases the predicted rubric score only when there is no grammatical error. Moreover, the model tolerates longer essays with only one grammatical error, which sounds quite reasonable. On the other side, the model finds desirable high numbers of sentence fragments, a non-trivial type of grammatical errors. Even worse, the model decreases the rubric score of essays having no sentence fragment. Although grammatical issues are beyond the scope of the “Style” rubric, the model has probably included these features because of their impact on the quality of assessment of vocabulary usage and sentence fluency. The reader should observe how the very poor quality of an essay can even prevent the detection of such fundamental grammatical errors such as in the case of Essay 228, where the AES tool did not find any grammatical error or sentence fragment. Therefore, there should be a way for AES systems to detect a minimum level of text quality before attempting to score an essay. Note that the objective of this section was not to undertake thorough debugging of the model, but rather to underscore the effectiveness of SHAP in doing so.

Formative Feedback

Once an AES model is considered reasonably valid, SHAP can be a suitable formalism to empower the machine to provide formative feedback. For instance, the explanation of Essay 124, which has been assigned a rubric score of 3 by both human and machine markers, indicates that the top two factors contributing to decreasing the predicted rubric score are: (1) the essay length being smaller than average, and (2) the average number of verb lemma types occurring at least once in the next sentence (adjacent_overlap_verb_sent). Figures 6A,H give the overall picture in which the realism of the contributions of these two features can be analyzed. More specifically, Essay 124 is one of very few essays ( Figure 6H ) that makes redundant usage of the same verbs across adjacent sentences. Moreover, the essay displays poor sentence fluency where everything is only expressed in two sentences. To understand more accurately the impact of “adjacent_overlap_verb_sent” on the prediction, a few spelling errors have been corrected and the text has been divided in four sentences instead of two. Revision 1 in Table 8 exhibits the corrections made to the original essay. The decision plot’s dashed line in Figure 3D represents the original explanation of Essay 124, while Figure 7A demonstrates the new explanation of the revised essay. It can be seen that the “adjacent_overlap_verb_sent” feature is still the second most important feature in the new explanation of Essay 124, with a feature value of 0.429, still considered as very poor according to the dependence plot in Figure 6H .

Table 8. Revisions of Essay 124: improvement of sentence splitting, correction of some spelling errors, and elimination of redundant usage of same verbs (bold for emphasis in Essay 124’s original version; corrections in bold for Revisions 1 and 2).

Figure 7. Explanations of the various versions of Essay 124 and evaluation of feature effect for a range of feature values. (A) Explanation of Essay 124’s first revision. (B) Forecasting the effect of changing the ‘adjacent_overlap_verb_sent’ feature on the rubric score. (C) Explanation of Essay 124’s second revision. (D) Comparison of the explanations of all Essay 124’s versions.

To show how SHAP could be leveraged to offer remedial formative feedback, the revised version of Essay 124 will be explained again for eight different values of “adjacent_overlap_verb_sent” (0, 0.143, 0.286, 0.429, 0.571, 0.714, 0.857, 1.0), while keeping the values of all other features constant. The set of these eight essays are explained by a newly trained SHAP explainer (Gradient), producing new SHAP values for each feature and each “revised” essay. Notice how the new model, called the feedback model, allows to foresee by how much a novice writer can hope to improve his/her score according to the “Style” rubric. If the student employs different verbs at every sentence, the feedback model estimates that the rubric score could be improved from 3.47 up to 3.65 ( Figure 7B ). Notice that the dashed line represents Revision 1, while other lines simulate one of the seven other altered essays. Moreover, it is important to note how changing the value of a single feature may influence the contributions that other features may have on the predicted score. Again, all explanations look similar in terms of direction, but certain features differ in terms of the magnitude of their contributions. However, the reader should observe how the targeted feature varies not only in terms of magnitude, but also of direction, allowing the student to ponder the relevancy of executing the recommended writing strategy.

Thus, upon receiving this feedback, assume that a student sets the goal to improve the effectiveness of his/her verb choice by eliminating any redundant verb, producing Revision 2 in Table 8 . The student submits his essay again to the AES system, which finally gives a new rubric score of 3.98, a significant improvement from the previous 3.47, allowing the student to get a 4 instead of a 3. Figure 7C exhibits the decision plot of Revision 2. To better observe how the various revisions of the student’s essay changed over time, their respective explanations have been plotted in the same decision plot ( Figure 7D ). Notice this time that the ordering of the features has changed to list the features of common importance to all of the essay’s versions. The feature ordering in Figures 7A−C complies with the same ordering as in Figure 3D , the decision plot of the original essay. These figures underscore the importance of tracking the interaction between the various features so that the model understands well the impact that changing one feature has on the others. TreeSHAP, an implementation for tree-based models, offers this capability and its potential on improving the quality of feedback provided to novice writers will be tested in a future version of this AES system.

This paper serves as a proof of concept of the applicability of XAI techniques in automated essay scoring, providing learning analytics practitioners and educators with a methodology on how to “hire” AI markers and make them accountable to their human counterparts. In addition to debug predictive models, SHAP explanation models can serve as some formalism of a broader learning analytics platform, where aspects of prescriptive analytics (provision of remedial formative feedback) can be added on top of the more pervasive predictive analytics.

However, the main weakness of the approach put forward in this paper consists in omitting many types of spatio-temporal data. In other words, it ignores precious information inherent to the writing process, which may prove essential to guess the intent of the student, especially in contexts of poor sentence structures and high grammatical inaccuracy. Hence, this paper calls for adapting current NLP technologies to educational purposes, where the quality of writing may be suboptimal, which is contrary to many utopian scenarios where NLP is used for content analysis, opinion mining, topic modeling, or fact extraction trained on corpora of high-quality texts. By capturing the writing process preceding a submission of an essay to an AES tool, other kinds of explanation models can also be trained to offer feedback not only from a linguistic perspective but also from a behavioral one (e.g., composing vs. revising); that is, the AES system could inform novice writers about suboptimal and optimal writing strategies (e.g., planning a revision phase after bursts of writing).

In addition, associating sections of text with suboptimal writing features, those whose contributions lower the predicted score, would be much more informative. This spatial information would not only allow to point out what is wrong and but also where it is wrong, answering more efficiently the question why an essay is wrong. This problem could be simply approached through a multiple-inputs and mixed-data feature-based (MLP) neural network architecture fed by both linguistic indices and textual data ( n -grams), where the SHAP explanation model would assign feature contributions to both types of features and any potential interaction between them. A more complex approach could address the problem through special types of recurrent neural networks such as Ordered-Neurons LSTMs (long short-term memory), which are well adapted to the parsing of natural language, and where the natural sequence of text is not only captured but also its hierarchy of constituents ( Shen et al., 2018 ). After all, this paper highlights the fact that the potential of deep learning can reach beyond the training of powerful predictive models and be better visible in the higher trustworthiness of explanation models. This paper also calls for optimizing the training of predictive models by considering the descriptive accuracy of explanations and the human expert’s qualitative knowledge (e.g., indicating the direction of feature contributions) during the training process.

Data Availability Statement

The datasets and code of this study can be found in these Open Science Framework’s online repositories: https://osf.io/fxvru/ .

Author Contributions

VK architected the concept of an ethics-bound, semi-autonomous, and trust-enabled human-AI fusion that measures and advances knowledge boundaries in human learning, which essentially defines the key traits of learning analytics. DB was responsible for its implementation in the area of explainable automated essay scoring and for the training and validation of the predictive and explanation models. Together they offer an XAI-based proof of concept of a prescriptive model that can offer real-time formative remedial feedback to novice writers. Both authors contributed to the article and approved its publication.

Research reported in this article was supported by the Academic Research Fund (ARF) publication grant of Athabasca University under award number (24087).

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Supplementary Material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/feduc.2020.572367/full#supplementary-material

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Keywords : explainable artificial intelligence, SHAP, automated essay scoring, deep learning, trust, learning analytics, feedback, rubric

Citation: Kumar V and Boulanger D (2020) Explainable Automated Essay Scoring: Deep Learning Really Has Pedagogical Value. Front. Educ. 5:572367. doi: 10.3389/feduc.2020.572367

Received: 14 June 2020; Accepted: 09 September 2020; Published: 06 October 2020.

Reviewed by:

Copyright © 2020 Kumar and Boulanger. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY) . The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

*Correspondence: David Boulanger, [email protected]

Disclaimer: All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article or claim that may be made by its manufacturer is not guaranteed or endorsed by the publisher.

A review of deep-neural automated essay scoring models

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• Published: 20 July 2021
• Volume 48 , pages 459–484, ( 2021 )

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• Masaki Uto   ORCID: orcid.org/0000-0002-9330-5158 1  

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Automated essay scoring (AES) is the task of automatically assigning scores to essays as an alternative to grading by humans. Although traditional AES models typically rely on manually designed features, deep neural network (DNN)-based AES models that obviate the need for feature engineering have recently attracted increased attention. Various DNN-AES models with different characteristics have been proposed over the past few years. To our knowledge, however, no study has provided a comprehensive review of DNN-AES models while introducing each model in detail. Therefore, this review presents a comprehensive survey of DNN-AES models, describing the main idea and detailed architecture of each model. We classify the AES task into four types and introduce existing DNN-AES models according to this classification.

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1 Introduction

Essay-writing tests have attracted much attention as a means to measuring practical and higher-order abilities such as logical thinking, critical reasoning, and creative thinking in various assessment fields (Abosalem 2016 ; Bernardin et al. 2016 ; Liu et al. 2014 ; Rosen and Tager 2014 ; Schendel and Tolmie 2017 ). In essay-writing tests, examinees write an essay about a given topic, and human raters grade those essays. However, essay grading is an expensive and time-consuming process, especially when there are many examinees (Hussein et al. 2019 ; Ke and Ng 2019 ). In addition, grading by human raters is not always consistent among and within raters (Eckes 2015 ; Hua and Wind 2019 ; Kassim 2011 ; Myford and Wolfe 2003 ; Rahman et al. 2017 ; Uto and Ueno 2018a ). One approach to resolving this problem is automated essay scoring (AES), which utilizes natural language processing (NLP) and machine learning techniques to automatically grade essays.

Many AES models have been developed over recent decades, and these can generally be classified as feature-engineering or automatic feature extraction approaches (Hussein et al. 2019 ; Ke and Ng 2019 ).

AES models based on the feature-engineering approach predict scores using textual features that are manually designed by human experts (e.g., Dascalu et al. 2017 ; Mark and Shermis 2016 ; Nguyen and Litman 2018 ). Typical features include essay length and the number of grammatical and spelling errors. The AES model first calculates these types of textual features from a target essay, then inputs the feature vector into a regression or classification model and outputs a score. Various models based on this approach have long been proposed (e.g., Nguyen and Litman 2018 ; Attali and Burstein 2006 ; Phandi et al. 2015 ; Beigman Klebanov et al. 2016 ; Cozma et al. 2018 ). For example, e-rater (Attali and Burstein 2006 ) is a representative model that was developed and has been used by the Educational Testing Service. Another recent popular model is the Enhanced AI Scoring Engine (Phandi et al. 2015 ), which achieved high performance in the Automated Student Assessment Prize (ASAP) competition run by Kaggle.

The advantages of feature-engineering approach models include interpretability and explainability. However, this approach generally requires extensive effort in engineering and tuning features to achieve high scoring accuracy for a target collection of essays. To obviate the need for feature engineering, automatic feature extraction approach models based on deep neural networks (DNNs) have recently attracted attention. Many DNN-AES models have been proposed over the last five years and have achieved state-of-the-art accuracy (e.g., Alikaniotis et al. 2016 ; Taghipour and Ng 2016 ; Dasgupta et al. 2018 ; Farag et al. 2018 ; Jin et al. 2018 ; Mesgar and Strube 2018 ; Wang et al. 2018 ; Mim et al. 2019 ; Nadeem et al. 2019 ; Uto et al. 2020 ; Ridley et al. 2021 ). The purpose of this paper is to review these DNN-AES models.

Several recent studies have reviewed AES models (Ke and Ng 2019 ; Hussein et al. 2019 ; Borade and Netak 2021 ). For example, Ke and Ng ( 2019 ) reviewed various AES models, including both feature-engineering approach models and DNN-AES models. However, because the purpose of their study was to present an overview of major milestones reached in AES research since its inception, they provided only a short summary of each DNN-AES model. Another review (Hussein et al. 2019 ) explained some DNN-AES models in detail, but only a few models were introduced. Borade and Netak ( 2021 ) also reviewed AES models, but they focused on feature-engineering approach models.

To our knowledge, no study has provided a comprehensive review of DNN-AES models while introducing each model in detail. Therefore, this review presents a comprehensive survey of DNN-AES models, describing the main idea and detailed architecture of each model. We classify AES tasks into four types according to recent findings (Li et al. 2020 ; Ridley et al. 2021 ), and introduce existing DNN-AES models according to this classification.

2 Automated essay scoring tasks

AES tasks are generally classified into the following four types (Li et al. 2020 ; Ridley et al. 2021 ).

Prompt-specific holistic scoring This is the most common AES task type, whereby an AES model is trained using rated essays that have holistic scores and have been written for a prompt. This trained model is used to predict the scores of essays written for the same prompt. Note that a prompt refers to an essay topic or a writing task that generally consists of reading materials and a task instruction.

Prompt-specific trait scoring This task involves predicting multiple trait-specific scores for each essay in a prompt-specific setting in which essays used for model training and unrated target essays are written for the same prompt. Such scoring is often required when an analytic rubric is used to provide more detailed feedback for educational purposes.

Cross-prompt holistic scoring In this task, an AES model is trained using rated essays with holistic scores written for non-target prompts and the trained model is transferred to a target prompt. This task has recently attracted attention because it is difficult to obtain a sufficient number of rated essays written for a target prompt in practice. This task includes a zero-shot setting in which rated essays written for a target prompt do not exist, and another setting in which a relatively small number of rated essays written for a target prompt can be used. The cross-prompt AES task relates to domain adaptation and transfer learning tasks, which are widely studied in machine learning fields.

Cross-prompt trait scoring This task involves predicting multiple trait-specific scores for each essay in a cross-prompt setting in which essays written for non-target prompts are used to train an AES model.

In the following section, we review representative DNN-AES models for each task type. Table  1 summarizes the models introduced in this paper.

3 Prompt-specific holistic scoring

This section introduces DNN-AES models for prompt-specific holistic scoring.

3.1 RNN-based model

One of the first DNN-AES models was a recurrent neural network (RNN)-based model proposed by Taghipour and Ng ( 2016 ). This model predicts a score for a given essay, defined as a sequence of words, by following multi-layered neural networks whose architecture is shown in Fig.  1 .

Architecture of RNN-based model

Lookup table layer This layer transforms each word in a given essay into a G -dimensional word-embedding representation. Word-embedding representation is a real-valued fixed-length vector of a word, in which words with similar meaning have similar vectors. Suppose \({{\mathcal {V}}}\) is a vocabulary list for essay collection, \(\varvec{w}_{t}\) represents a \(|{{\mathcal {V}}}|\) -dimensional one-hot representation of t -th word \(w_{t}\) in a given essay, and \(\varvec{A}\) represents a \(G \times |{{\mathcal {V}}}|\) -dimensional trainable embeddings matrix. Then, the embedding representation \(\tilde{\varvec{w}}_t\) corresponding to \(w_{t}\) is calculable as a dot product \(\tilde{\varvec{w}}_t = \varvec{A}\cdot \varvec{w}_{t}\) .

Convolution layer This layer captures local textual dependencies using convolution neural networks (CNNs) from a sequence of word-embedding vectors. Given an input sequence \(\{\tilde{\varvec{w}}_1,\tilde{\varvec{w}}_2, \ldots , \tilde{\varvec{w}}_L\}\) (where L is the number of words in a given essay), this layer is applied to a window of c words to capture local textual dependencies among c -gram words. Concretely, the t -th output of this layer is calculable as follows.

where \(\mathbf {W_c}\) and \(b_c\) are trainable weight and bias parameters, and \([\cdot , \cdot ]\) means the concatenation of the given elements. Zero padding is applied to outputs from this layer to preserve the input and output sequence lengths. This is an optional layer that has often been omitted in recent studies.

Recurrent layer This layer generally uses a long short-term memory (LSTM) network, a representative RNN, that outputs a vector at each timestep while capturing time series dependencies in an input sequence. A single-layer unidirectional LSTM is generally used, but bidirectional or multilayered LSTMs are also often used.

Pooling layer This layer transforms the output hidden vector sequence of the recurrent layer \(\{ \varvec{h}_{1}, \varvec{h}_{2}, \ldots ,\varvec{h}_{L}\}\) (where \(\varvec{h}_{t}\) represents the hidden vector of the t -th output of the recurrent layer) into an aggregated fixed-length hidden vector. Mean-over-time pooling, which calculates an average vector

is generally used because it tends to provide stable accuracy. Other frequently used pooling methods include the last pool (Alikaniotis et al. 2016 ), which uses the last output of the recurrent layer \(\varvec{h}_{L}\) , and an attention pooling layer (Dong et al. 2017 ), which we explain later in the present study.

Linear layer with sigmoid activation This layer projects a pooling layer output onto a scalar value in the range [0, 1] by utilizing the sigmoid function as

where \(\mathbf{W}_o\) is a weight matrix and \(b_o\) represents bias parameters. \(\sigma ()\) represents the sigmoid function.

For model training, the mean-squared error (MSE) between predicted and gold-standard scores is generally used as the loss function. Specifically, letting \(y_{n}\) be the gold-standard score for n -th essay and letting \({\hat{y}}_{n}\) be the predicted score, the MSE loss function is defined as

where N is the number of essays. Note that the model training is conducted after normalizing gold standard scores to [0, 1], but the predicted scores are linearly rescaled to the original score range in the prediction phase.

3.2 RNN-based model with score-specific word embedding

Alikaniotis et al. ( 2016 ) also proposed a similar RNN-based model consisting of three layers, namely, a lookup table layer, a recurrent layer, and a pooling layer. The model uses a bidirectional LSTM for the recurrent layer and the last pooling for the pooling layer. The unique feature of this model is the use of score-specific word embedding (SSWE), which is an extension of Collobert & Weston (C&W) word-embedding (Collobert and Weston 2008 ), in the lookup table layer.

Suppose we train a representation for a target word \(w_t\) within a sequence of one-hot encoded words \(\varvec{S}=\{ \varvec{w}_1, \ldots , \varvec{w}_t, \ldots \varvec{w}_L\}\) . To derive this representation, the C&W word-embedding model learns to distinguish between the original sequence \(\varvec{S}\) and an artificially created noisy sequence \(\varvec{S}'\) in which the target word is substituted for a randomly selected word. Given a trainable embedding matrix \(\varvec{A}\) , the model concatenates the embedding representation vectors of the words in the sequence, that is, \(\tilde{\varvec{S}} = [\varvec{A} \cdot \varvec{w}_1, \varvec{A} \cdot \varvec{w}_2, \ldots , \varvec{A} \cdot \varvec{w}_L]\) . Using the vector, the C&W word-embedding model predicts whether the given word sequence \(\varvec{S}\) is the original sequence or a noisy one based on the following function.

where \(\mathbf{W}_1\) , \(\mathbf{W}_2\) , \(b_1\) , and \(b_2\) are the trainable parameters, and \(\mathrm {htanh}()\) is the hard hyperbolic tangent function.

The SSWE model extends the C&W word-embedding model by adding another output layer that predicts essay scores as follows.

where \(\mathbf{W}'_1\) , \(\mathbf{W}'_2\) , \(b'_1\) , and \(b'_2\) are the trainable parameters. The SSWE model is trained while minimizing a weighted linear combination of two error loss functions, namely, a classification loss function based on Eq. ( 5 ) and a scoring error loss function based on Eq. ( 6 ).

The SSWE model provides a more effective word-embedding representation to distinguish essay qualities than does the C&W word-embedding model. Thus, Alikaniotis et al. ( 2016 ) proposed using the embedding matrix \(\varvec{A}\) trained by the SSWE model in the lookup table layer.

3.3 Hierarchical representation models

Architecture of hierarchical representation model

The models introduced above handle an essay as a linear sequence of words. Dong and Zhang ( 2016 ), however, proposed modeling the hierarchical structure of a text. Concretely, they assumed that an essay is constructed as a sequence of sentences defined as word sequences. Accordingly, they introduced a two-level hierarchical representation model consisting of a word-level CNN and a sentence-level CNN, as shown in Fig.  2 . Each CNN works as explained below.

Word-level CNN The sequence of words in each sentence is processed and an aggregated vector is output, which can be taken as an embedding representation of a sentence. Suppose an essay consists of I sentences \(\{\varvec{s}_1, \ldots , \varvec{s}_I\}\) , and each sentence is defined as a sequence of words as \(\varvec{s}_i = \{w_{i1}, \ldots , w_{iL_i}\}\) (where \(w_{it}\) is the t -th word in i -th sentence, and \(L_i\) is the number of words in i -th sentence). For each sentence \(\varvec{s}_i\) , the lookup table layer transforms each word into an embedding representation, and then the word-level CNN processes the sequence of word-embedding vectors. The operation of the word-level CNN is the same as that of the convolution layer explained in Subsection  3.1 . The output sequence of the word-level CNN is transformed into an aggregated fixed-length hidden vector \(\tilde{\varvec{h}}_{s_i}\) through a pooling layer.

Sentence-level CNN This CNN takes the sequence of sentence vectors \(\{\tilde{\varvec{h}}_{s_1}, \ldots , \tilde{\varvec{h}}_{s_I}\}\) as input and extracts n-gram level features over the sentence sequence. Then, a pooling layer transforms the CNN output sequence into an aggregated fixed-length hidden vector \(\tilde{\varvec{h}}\) . Finally, the linear layer with sigmoid activation maps vector \(\tilde{\varvec{h}}\) to a score.

Dong et al. ( 2017 ) proposed another hierarchical representation model that extends the above model by using an attention mechanism (Bahdanau et al. 2014 ) to automatically identify important words and sentences. The attention mechanism is a neural architecture that enables to dynamically focus on relevant regions of input data to make predictions. The main idea of the attention mechanism is to compute a weight distribution on the input data, assigning higher values to more relevant regions. (Dong et al. 2017 ) uses attention-based pooling in the pooling layers. Letting the input sequence for the pooling layer be \(\{\varvec{x}_1, \ldots , \varvec{x}_J\}\) , where J indicates the sequence length, the attention mechanism aggregates the input sequence into a fixed-length vector \(\tilde{\varvec{x}}\) by performing the following operations.

In these equations, \(\mathbf{W}_{a_1}\) , \(\mathbf{W}_{a_2}\) , and b are trainable parameters. \(\tilde{\varvec{x}}_j\) and \(a_j\) are called an attention vector and an attention weight for j -th input, respectively.

In addition to the incorporation of the attention mechanism, Dong et al. ( 2017 ) proposed adding a character-level CNN before the word-level CNN and using LSTM as an alternative to the sentence-level CNN.

3.4 Coherence modeling

Coherence is an important criterion for evaluating the quality of essays. However, the RNN-based models introduced above are known to have difficulty capturing the relationships between multiple regions in an essay because they compress a word sequence within a fixed-length hidden vector in the order they are inputted. To resolve this difficulty, several DNN-AES models that consider coherence features have been proposed (Tay et al. 2018 ; Li et al. 2018 ; Farag et al. 2018 ; Mesgar and Strube 2018 ; Yang and Zhong 2021 ). This subsection introduces two representative models.

3.4.1 SKIPFLOW model

Architecture of SKIPFLOW model

Tay et al. ( 2018 ) proposed SKIPFLOW, which learns coherence features explicitly using a neural network architecture. The model is the RNN-based model with a neural tensor layer as shown in Fig.  3 . The neural tensor layer takes two positional outputs of the recurrent layer that are collected from different time steps as input and computes the similarity between each of these pairs of positional outputs. Concretely, for a recurrent layer output sequence \(\{ \varvec{h}_{1}, \varvec{h}_{2}, \ldots ,\varvec{h}_{L}\}\) , the model first selects a pair of sequential outputs of width \(\delta\) , that is, \(\{(\varvec{h}_1,\varvec{h}_\delta ), (\varvec{h}_{\delta +1},\varvec{h}_{2\delta }), \ldots , (\varvec{h}_{t\delta +1}, \varvec{h}_{(t+1)\delta }), \ldots \}\) . Then, each pair of hidden vectors \((\varvec{h}_{t\delta +1}, \varvec{h}_{(t+1)\delta })\) is input into the following neural tensor layer to return a similarity score as

where \(\mathbf{W}_u\) , \(\mathbf{V}\) , and \(\mathbf{b}_u\) are the weight and bias vectors and \(\mathbf{M}\) is a three-dimensional tensor. These are trainable parameters.

The similarity scores for all the pairs are concatenated with the polling layer output vector \(\tilde{\varvec{h}}\) as

and the resulting vector is mapped to a score through a fully connected neural network layer and a linear layer with sigmoid activation.

3.4.2 Self-attention-based model

Li et al. ( 2018 ) proposed another model using a self-attention mechanism to capture relationships between multiple points in an essay. Self-attention mechanisms have been shown to be able to capture long-distance relationships between words in a sequence and have recently been used in various NLP tasks.

Architecture of self-attention-based model

Figure  4 shows the model architecture. This model first transforms each word into an embedding representation through a lookup table layer with a position encoding, and then inputs the sequence into a multi-head self-attention model that combines multiple self-attention models in parallel. See Vaswani et al. ( 2017 ) for details of the lookup table layer with the position encoding and the multi-head self-attention architecture. The self-attention output sequence is input into a recurrent layer, a pooling layer, and a linear layer with sigmoid activation to produce an essay score.

3.5 BERT-based models

Bidirectional encoder representations from transformers (BERT), a pre-trained language model released by the Google AI Language team in 2018, has achieved state-of-the-art results in various NLP tasks (Devlin et al. 2019 ). Since then, BERT has also been applied to automated text scoring tasks, including AES (Nadeem et al. 2019 ; Uto et al. 2020 ; Rodriguez et al. 2019 ; Yang et al. 2020 ; Mayfield and Black 2020 ) and automated short-answer grading (Liu et al. 2019 ; Lun et al. 2020 ; Sung et al. 2019 ), and has shown good performance.

BERT is defined as a multilayer bidirectional transformer network (Vaswani et al. 2017 ). Transformers are a neural network architecture designed to handle ordered sequences of data using an attention mechanism. Specifically, transformers consist of multiple layers (called transformer blocks), each containing a multi-head self-attention network and a position-wise fully connected feed-forward network. See (Vaswani et al. 2017 ) for details of this architecture.

Architecture of BERT-based model

Architecture of BERT-based model with ranking task

BERT is trained in pre-training and fine-tuning steps. Pre-training is conducted on huge amounts of unlabeled text data over two tasks, namely, masked language modeling and next-sentence prediction. Masked language modeling is the task that predicts the identities of words that have been masked out of the input text. Next-sequence prediction is the task that predicts whether two given sentences are adjacent.

Using pre-trained BERT for a target NLP task, such as AES, requires fine-tuning (retraining), which is conducted from a task-specific supervised dataset after initializing model parameters to pre-trained values. When using BERT for AES, input essays require preprocessing, namely, adding a special token (“CLS”) to the beginning of each input. BERT output corresponding to this token is used as the aggregate hidden representation for a given essay (Devlin et al. 2019 ). We can thus score an essay by inputting its representation into a linear layer with sigmoid activation, as illustrated in Fig.  5 .

Furthermore, Yang et al. ( 2020 ) proposed fine-tuning the BERT model so that the essay scoring task and an essay ranking task are jointly resolved. As shown in Fig.  6 , the proposed model is formulated as a BERT-based AES model with an additional output layer that predicts essay ranks. The model uses ListNet (Cao et al. 2007 ) for predicting the ranking list. This model is fine-tuned by minimizing a combination of the scoring MSE loss function and a ranking error loss function based on ListNet.

3.6 Hybrid models

Architecture of hybrid model with additional RNN for sentence-level features

Architecture of DNN-AES with handcrafted essay-level features

The feature-engineering approach and the DNN-AES approach can be viewed as complementary rather than competing approaches (Ke and Ng 2019 ; Uto et al. 2020 ) because they provide different advantages. To receive both benefits, some hybrid models that integrate the two approaches have been proposed (Dasgupta et al. 2018 ; Uto et al. 2020 ).

One of the hybrid models is proposed by Dasgupta et al. ( 2018 ). Figure  7 shows the model architecture. As shown in the figure, it mainly consists of two DNNs. One processes word sequences in a given essay in the same way as the conventional RNN-based model (Taghipour and Ng 2016 ). Specifically, a word sequence is transformed into a fixed-length hidden vector \(\tilde{\varvec{h}}\) through a lookup table layer, a convolution layer, a recurrent layer, and a pooling layer. The other DNN processes a sequence of manually designed sentence-level features. Letting a given essay have I sentences, and letting \(\varvec{f}_{i}\) be a manually designed sentence-level feature vector for i -th sentence, the feature sequence \(\{\varvec{f}_{1},\varvec{f}_{2},\ldots ,\varvec{f}_{I}\}\) is transformed into a fixed-length hidden vector \(\tilde{\varvec{h}}_f\) through a convolution layer, a recurrent layer, and a pooling layer. The model uses LSTM for the recurrent layer and attention pooling for the pooling layer. Finally, after concatenating the hidden vectors \([\tilde{\varvec{h}}, \tilde{\varvec{h}}_f]\) , a linear layer with sigmoid activation maps it to a score.

Another hybrid model is formulated as a DNN-AES model incorporating manually designed essay-level features (Uto et al. 2020 ). Concretely, letting \(\varvec{F}\) be a manually designed essay-level feature vector, the model concatenates the feature vector with the hidden vector \(\tilde{\varvec{h}}\) , which is obtained from a DNN-AES model. Then, a linear layer with sigmoid activation maps the concatenated vector \([\tilde{\varvec{h}}, \varvec{F}]\) to a score value. Figure  8 shows the architecture of this model. This hybrid model is easy to construct using various DNN-AES models.

3.7 Improving robustness for biased training data

DNN-AES models generally require a large dataset of essays graded by human raters as training data. When creating a training dataset, essay grading tasks are generally shared among many raters by assigning a few raters to each essay to lower the burden of assessment. However, in such cases, assigned scores are known to be biased owing to the effects of rater characteristics (Rahman et al. 2017 ; Amidei et al. 2020 ). The performance of AES models drops when biased data are used for model training because the resulting model reflects the bias effects (Amorim et al. 2018 ; Huang et al. 2019 ; Li et al. 2020 ).

To resolve this problem, Uto and Okano ( 2020 ) proposed an AES framework that integrates item response theory (IRT), a test theory based on mathematical models. Specifically, they used an IRT model incorporating parameters representing rater characteristics (e.g., Eckes 2015 ; Uto and Ueno 2016 , 2018a ) that can estimate essay scores while mitigating rater bias effects. The applied IRT model is the generalized many-facet Rasch model (Uto and Ueno 2018b , 2020 ) that defines the probability that rater r assigns score k to n -th essay for a prompt as

where \(\alpha _r\) is the consistency of rater r , \(\beta _{r}\) is the severity of rater r , \(\beta _{rm}\) represents the strictness of rater r for category m , and K indicates the number of score categories. Furthermore, \(\theta _n\) represents the latent scores for n -th essay, which removes the effects of the rater characteristics.

Using this IRT model, Uto and Okano ( 2020 ) proposed training an AES model through the following two steps. 1) Apply the IRT model to observed rating data to estimate the IRT-based score \(\theta _n\) , which removes the effects of rater bias. 2) Train an AES model using the unbiased scores \(\varvec{\theta } =\{\theta _1, \ldots , \theta _N\}\) as the gold-standard scores based on the following loss function.

where \({\hat{\theta }}_n\) represents the AES’s predicted score for n -th essay. Because the IRT-based scores are theoretically free from rater bias effects, the AES model will not reflect the bias effects.

In the prediction phase, the score for a new essay is calculated in two steps: (1) Predict the IRT score \(\theta\) for the essay using a trained AES model. (2) Given \(\theta\) and rater parameters, calculate the expected score, which corresponds to an unbiased original-scaled score (Uto 2019 ), as

where R indicates the number of raters who graded essays in the training data. The expected score is used as a predicted essay score, which is robust against rater biases.

3.8 Integration of AES models

Conventional AES models including those introduced above have different scoring characteristics. Therefore, integrating multiple AES models is expected to improve scoring accuracy. For these reasons, Aomi et al. ( 2021 ) proposed a framework that integrates multiple AES models while considering the characteristics of each model using IRT. In the framework, multiple AES models are first trained independently, and the trained models are used to produce prediction scores for target essays. Then, the generalized many-facet Rasch model introduced above is applied to the obtained prediction scores by regarding rater characteristic parameters, \(\alpha _r\) , \(\beta _r\) , and \(\beta _{rm}\) as characteristic parameters of AES models. Given the estimated IRT score \(\theta\) for the target essays, a predicted essay score is calculated as the expected score based on Eq. ( 12 ).

This framework can integrate prediction scores from various AES models while considering the characteristics of each model. Subsequently, it provides scores that are more accurate than those obtained by simple averaging or a single AES model.

4 Prompt-specific trait scoring

This section introduces DNN-AES models for the prompt-specific trait scoring task. Although this task is important especially for educational purposes, only a limited number of models have been proposed for the task.

4.1 Use of multiple trait-specific models

Mathias et al. ( 2020 ) presents one of the first attempts to perform prompt-specific trait scoring based on a DNN-AES model. Their study used the hierarchical representation model with an attention mechanism (Dong et al. 2017 ), introduced in Sect.  3.3 , to predict trait-specific scores for each essay. Concretely, in their study, the AES model was trained for each trait independently, and predicted scores using trait-specific models.

Architecture of RNN-based model with multiple output layers for prompt-specific trait scoring

4.2 Model with multiple output modules

Hussein et al. ( 2020 ) proposed a model specialized in a prompt-specific trait scoring task that can predict multiple trait scores jointly. The model is formulated as a multi-output model based on the RNN-based model (Taghipour and Ng 2016 ), introduced in Sect.  3.1 . Concretely, as shown in Fig.  9 , they extended the RNN-based model by adding as many multiple output linear layers as the number of traits. Additionally, an optional fully connected neural network layer was added after the pooling layer. The loss function is defined as a linear combination of multiple MSE loss functions as follows.

where D is the number of traits, \(y_{nd}\) and \({\hat{y}}_{nd}\) are the gold-standard score and predicted d -th trait score for n -th essay, respectively.

5 Cross-prompt holistic scoring

The prompt-specific scoring models introduced above assume situations in which rated training essays and unrated target essays are written for the same prompt. However, we often face situations in which we cannot use any rated essays or only a relatively small number of rated essays written for the target prompt in model training, even though we have many rated essays written for other non-target prompts. AES for such settings is generally called a cross-prompt scoring task. This section introduces cross-prompt holistic scoring models.

5.1 Two-stage learning models

One of the first cross-prompt holistic scoring models using DNN was proposed by Jin et al. ( 2018 ). The method is constructed as a two-stage DNN (TDNN) approach in which a prompt-independent scoring model is trained using rated essays for non-target prompts in the first stage, and is used to generate pseudo rating data for unrated essays in a target prompt. Then, using the pseudo rating data, a prompt-specific scoring model for the target prompt is trained in the second stage. The TDNN is detailed below.

First stage (Training a prompt-independent AES model) In this stage, rated essays written for non-target prompts are used to train a prompt-independent AES model that uses manually designed prompt-independent shallow features, such as the number of typos, grammatical errors, and spelling errors. Here, a ranking support vector machine (Joachims 2002 ) is used as the prompt-independent model.

Second stage (Training a prompt-specific AES model) The trained prompt-independent AES model is used to produce the scores of unrated essays written for a target prompt, and the pseudo scores are used to train a prompt-specific scoring model. To train a prompt-specific scoring model, only confident essays with the highest and lowest pseudo scores are used, instead of using all the produced scores. The prompt-specific AES model in the study by Jin et al. ( 2018 ) used an extended model of the RNN-based model (Taghipour and Ng 2016 ) that can process three types of sequential inputs, namely, a sequence of words, part-of-speech (POS) tags, and syntactic tags.

Li et al. ( 2020 ) pointed out that the TDNN model uses a limited number of general linguistic features in the prompt-independent AES model, which may seriously affect the accuracy of the generated pseudo scores for essays in a target prompt. To extract more efficient features, they proposed another two-stage framework called a shared and enhanced deep neural network (SEDNN) model. The SEDNN model consists of two stages, described as follows.

First stage As an alternative to a prompt-independent model with manually designed shallow linguistic features, the SEDNN uses a DNN-AES model that extends the hierarchical representation model with an attention mechanism (Dong et al. 2017 ), introduced in Sect.  3.3 . Concretely, in the model, a new output layer is added to jointly solve the AES task and a binary classification task that distinguishes whether a given essay was written for the target prompt. The model is trained based on a combination of the loss functions for the essay scoring task and the prompt discrimination task using a dataset consisting of rated essays written for non-target prompts and the unrated essays written for the target prompt.

Second stage As in the second stage of the TDNN model, scores of unrated essays written for a target prompt are generated by the prompt-independent AES model, and the pseudo scores are used to train a prompt-specific scoring model. The prompt-specific scoring model in the study by Li et al. ( 2020 ) is a Siamese network model that jointly uses the essay text and the text of the target prompt itself to learn prompt-dependent features more efficiently. In the model, an essay text is processed by a similar model to the SKIPFLOW (Tay et al. 2018 ) and is transformed into vector representations. The word sequence in the prompt text is also transformed into a fixed-length hidden vector representation by another neural architecture consisting of a lookup table layer, a convolution layer, a recurrent layer, and a mechanism that measures the relevance relation between the given essay and the target prompt text. After concatenating the two vector representations corresponding to an essay text and a prompt text, a linear layer with sigmoid activation maps it to a prediction score.

5.2 Multi-stage pre-training approach model

Another cross-prompt holistic scoring approach incorporates pre-training processes. In the approach, an AES model is developed by performing pre-training on a vast number of essays with or without scores written for non-target prompts, and then the model is fine-tuned using a limited number of rated essays written for a target prompt. The pre-training process enables a DNN model to capture a general language model for predicting essay quality. Thus, the use of a pre-trained model as an initial model helps in obtaining a model for a target scoring task. The BERT-based AES models explained in Sect.  3.5 are examples of the pre-training and fine-tuning approach models. In various NLP tasks, the use of pre-training has been popular and has achieved great success.

For cross-prompt holistic scoring, (Song et al. 2020 ) proposed training the hierarchical representation model with the attention mechanism (Dong et al. 2017 ), as explained in Sect.  3.3 , through the following three pre-training and fine-tuning steps.

Weakly supervised pre-training The AES model is trained based on a vast number of roughly scored essays written for diverse prompts collected from the Web. The study by Song et al. ( 2020 ) assumed that binary scores are given to the essays; thus, this step is called weakly supervised. The objective of this pre-training step was to have the AES model learn a general language representation that can roughly distinguish essay quality.

Cross-prompt supervised fine-tuning If we have rated essays written for non-target prompts, the pre-trained model is fine-tuned using the data.

Target-prompt supervised fine-tuning The model obtained from the above steps is fine-tuned using rated essays written for the target prompt. The study by Song et al. ( 2020 ) reported that incorporating the above two-stage pre-training and fine-tuning improves the performance of the target-prompt scoring.

5.3 Model with self-supervised learning

Cao et al. ( 2020 ) proposed another cross-prompt holistic scoring model that was designed to solve the AES task with two prompt-independent self-supervised learning tasks jointly. The two self-supervised learning tasks, which are appended to efficiently extract prompt-independent common knowledge, are a sentence reordering task and a noise identification task , as explained bellow.

Sentence reordering In this task, each essay is divided into four parts and then shuffled according to a certain permutation order. The sentence reordering task predicts an appropriate permutation for each given essay.

Noise identification In this task, each essay is transformed into noisy data by performing random insertion, random swap, and random deletion operations on 10% of the words in the essay. The noise identification task predicts whether a given essay is noisy or not.

The above two self-supervised learning tasks are simultaneously trained with the AES task in a model. Figure  10 shows the model architecture. This model has a shared encoder that transforms an input word sequence into a fixed-length essay representation vector, and three task-specific output layers.

Architecture of cross-prompt holistic scoring model with self-supervised learning

The shared encoder is formulated as a hierarchical representation DNN model such as that introduced in Sect.  3.3 . In this model, a sequence of words corresponding to each sentence is transformed into a fixed-length sentence representation vector through a lookup table layer, a recurrent layer, a self-attention layer, a fusion gate, and a mean-over-time pooling layer. Here, the fusion gate is an operation that combines the input and output of the self-attention layer as follows.

where \(\varvec{H}_{s_i}\) and \(\tilde{\varvec{H}}_{s_i}\) are the input and output vector sequences of the self-attention layer for i -th sentence, and \(\hat{\varvec{H}}_{s_i}\) is the fusion gate output. \(\mathbf{W}_{g1}\) and \(\mathbf{W}_{g2}\) are trainable parameters. The essay representation vector is calculated by averaging the obtained sentence vectors, and the vector is used for the AES and the two self-supervised learning tasks. This model is trained based on a weighted sum of the MSE loss function for the AES and error loss functions for the two self-supervised learning tasks.

Furthermore, Cao et al. ( 2020 ) proposed a technique to improve the adaptability of the model to a target prompt. Concretely, during the model training processes, this technique calculates the averaged essay representation vector for each prompt and shifts the representation of each essay into the target prompt’s averaged vector.

6 Cross-prompt trait scoring

This section introduces cross-prompt trait scoring models that predict multiple trait-specific scores for each essay in a cross-prompt setting.

6.1 Use of multiple trait-specific models with self-supervised learning

Mim et al. ( 2019 ) proposed a method to predict two trait scores, namely, coherence and argument strength , for each essay. They used a vast number of unrated essays for non-target prompts to pre-train a DNN model, and then the model was transferred to a target AES task. They used the RNN-based model (Taghipour and Ng 2016 ) introduced in Sect.  3.1 as the base model. The detailed processes are as follows.

Pre-training based on self-supervised learning with non-target essays In this step, the base model is trained using unrated essays written for non-target prompts based on a self-supervised learning task, which is a binary classification task that distinguishes artificially created incoherent essays. For the self-supervised learning task, incoherent essays are created by randomly shuffling sentences, discourse indicators, and paragraphs in the original essays. This pre-training is introduced to enable the base model to learn features for distinguishing logical text from illogical text.

Pre-training based on self-supervised learning with target essays The pre-trained model is retrained using essays written for the target prompt based on the same self-supervised task described above. This step is introduced to alleviate mismatch between essays written for non-target prompts and those written for the target prompt.

Fine-tuning for AES The pre-trained model is fine-tuned for the AES task using rated essays for the target prompt. Note that, for the AES task, the base model is extended by adding two RNN-based architectures that process a prompt text and a sequence of paragraph function labels (i.e., Introduction, Body, Rebuttal and Conclusion). The fine-tuning is conducted independently for two traits, namely, coherence and argument strength .

6.2 Model with multiple output modules

Ridley et al. ( 2021 ) proposed a model specialized in trait scoring that can predict multiple trait scores jointly. As shown in Fig.  11 , the model is formulated as the following multi-output DNN model.

Architecture of cross-prompt trait scoring model with multiple output layers

Shared layers The model first processes an input through shared layers that is commonly used for predicting all trait scores. The shared layers consist of a POS embedding layer, a convolutional layer, and an attention pooling layer, as explained below.

A POS embedding layer takes a sequence of POS tags for words in a given essay and transforms it into embedding representations, using the same operations as in the lookup table layer. Note that this model uses a POS tag sequence as the input instead of a word sequence because word information depends strongly on a prompt, but POS information that represents syntactic information is more adaptable to different prompts.

A convolutional layer extracts n-gram level features from a sequence of POS embeddings for each sentence in the same way as described in Sect.  3.1 .

An attention pooling layer applies an attention mechanism to produce a fixed-length vector representation for each sentence from the convolutional layer outputs.

Trait-specific layers The sequence of sentence representations produced by the shared convolutional layer is input into trait-specific layers that are used for predicting each trait score through the following procedures.

The sentence representation sequence is transformed into a fixed-length vector corresponding to essay representation through a recurrent layer, and an attention pooling layer.

The essay representation vector is concatenated with prompt-independent manually designed features, similar to those used in the first stage of TDNN.

To obtain a final representation for each trait score, the model applies an attention mechanism so that each trait-specific layer can utilize the relevant information from the other trait-specific layers.

A linear layer with sigmoid activation maps the aggregated vector to a corresponding trait score.

The loss function for training this model is similar to Eq. ( 13 ). Note that, because different prompts are often designed to evaluate different trait scores, the model introduces a masking function. Concretely, letting \(mask_{nd}\) be a variable that takes 1 if the prompt corresponding to n -th essay that has d -th trait score, and 0 otherwise, the loss function with the mask function is defined as follows.

The mask function enables the loss values for the traits without the gold scores to be 0.

A special case of this model that has a single output layer for the holistic score has also been proposed as a cross-prompt holistic scoring model (Ridley et al. 2020 ).

7 Conclusions and remarks

This review has presented a comprehensive survey of DNN-AES models. Concretely, we classified the AES task into four types, namely, (1) prompt-specific holistic scoring, (2) prompt-specific trait scoring, (3) cross-prompt holistic scoring, and (4) cross-prompt trait scoring, and introduced the main ideas and the architectures of representative DNN-AES models for each task type.

As shown in our study, earlier DNN-AES models focus mainly on the prompt-specific holistic scoring task. The commonly used baseline model is the RNN-based model (Taghipour and Ng 2016 ), which has been extended by incorporating an efficient word embedding representation, a hierarchical structure of a text, a coherence model, and manually designed features. We also described transformer-based models such as BERT that have recently been applied to AES with their widespread use in various machine learning research studies.

These prompt-specific holistic scoring models have been extended for prompt-specific trait scoring, which predicts multiple trait scores for each essay. Trait scoring is practically important, especially when we need to provide detailed feedback to examinees for educational purposes, although the number of papers for this task is still limited.

Although prompt-specific scoring tasks assume that we can use a sufficient number of rated essays for a target prompt, this assumption is not often satisfied in practice because collecting rated essays is an expensive and time-consuming task. To overcome this limitation, cross-prompt scoring models have provided frameworks that use a large number of essays for non-target prompts. Although the number of cross-prompt scoring models is still limited, this task is important for increasing the feasibility of applying DNN-AES models to practical situations.

We can use several corpora to develop and evaluate AES models. ASAP corpus, which was released as part of a Kaggle competition, has been commonly used in holistic scoring models. For the trait scoring models, the International Corpus of Learner English (Ke 2019 ) and ASAP++ corpus (Mathias et al. 2018 ) are available. See (Ke and Ng 2019 ) for a more detailed summary of these corpora.

A future direction of AES studies is developing efficient and accurate trait scoring models and cross-prompt models. As described above, although the number of studies for those DNN-AES models is limited, such studies are essential to the use of AES technologies in various situations. It is also important to develop methodologies that reduce costs and noise when training data are being created. Approaches to reducing rating costs include recently examined active learning approaches (e.g., Hellman et al. 2019 ). To reduce scoring noise or biases, the integration of statistical models such as the IRT models described in Sect.  3.7 would be a possible approach.

Another future direction is to analyze the quality of each essay test and the characteristics of an applied AES model based on test theory. From the perspective of test theory, evaluating the reliability and validity of a test and its scoring processes is important for discussing the appropriateness of the test as a measurement tool. Although AES studies tend to ignore these points, several works have considered the relationship between DNN-based AES tasks and test theory (e.g., Uysal and Doğan 2021 ; Uto and Uchida 2020 ; Ha et al. 2020 ).

The application of AES methods to various related domains is also desired. For example, AES methods would be applicable to various operations such as writing support systems (e.g., Ito et al. 2020 ; Tsai et al. 2020 ) and peer grading processes (Han et al. 2020 ).

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Acknowledgements

This work was supported by JSPS KAKENHI Grant Numbers 19H05663 and 21H00898.

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Uto, M. A review of deep-neural automated essay scoring models. Behaviormetrika 48 , 459–484 (2021). https://doi.org/10.1007/s41237-021-00142-y

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Received : 18 June 2021

Accepted : 08 July 2021

Published : 20 July 2021

Issue Date : July 2021

DOI : https://doi.org/10.1007/s41237-021-00142-y

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