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MRQA 2019 Shared Task: Evaluating Generalization in Reading Comprehension

We present the results of the Machine Reading for Question Answering (MRQA) 2019 shared task on evaluating the generalization capabilities of reading comprehension systems. In this task, we adapted and unified 18 distinct question answering datasets into the same format. Among them, six datasets were made available for training, six datasets were made available for development, and the rest were hidden for final evaluation. Ten teams submitted systems, which explored various ideas including data sampling, multi-task learning, adversarial training and ensembling. The best system achieved an average F1 score of 72.5 on the 12 held-out datasets, 10.7 absolute points higher than our initial baseline based on BERT.


Introduction
Machine Reading for Question Answering (MRQA) has become an important testbed for evaluating how well computer systems understand human language. Interest in MRQA settings-in which a system must answer a question by reading one or more context documents-has grown rapidly in recent years, fueled especially by the creation of many large-scale datasets (Rajpurkar et al., 2016;Joshi et al., 2017;Kwiatkowski et al., 2019). MRQA datasets have been used to benchmark progress in general-purpose language understanding (Devlin et al., 2018;Yang et al., 2019). Interest in MRQA also stems from their use in industry applications, such as search engines (Kwiatkowski et al., 2019) and dialogue systems (Reddy et al., 2019;Choi et al., 2018).
While recent progress on benchmark datasets has been impressive, MRQA systems are still primarily evaluated on in-domain accuracy. It remains challenging to build MRQA systems that 1 https://github.com/mrqa/MRQA-Shared- Task-2019. generalize to new test distributions Levy et al., 2017;Yogatama et al., 2019) and are robust to test-time perturbations (Jia and Liang, 2017;Ribeiro et al., 2018). A truly effective question answering system should do more than merely interpolate from the training set to answer test examples drawn from the same distribution: it should also be able to extrapolate to test examples drawn from different distributions.
In this work we introduce the MRQA 2019 Shared Task on Generalization, which tests extractive question answering models on their ability to generalize to data distributions different from the distribution on which they were trained. Ten teams submitted systems, many of which improved over our provided baseline systems. The top system, which took advantage of newer pre-trained language models (Yang et al., 2019;, achieved an average F1 score of 72.5 on our hidden test data, an improvement of 10.7 absolute points over our best baseline. Other submissions explored using adversarial training, multitask learning, and better sampling methods to improve performance. In the following sections, we present our generalization-focused, extractive question-answering dataset, a review of the official baseline and participating shared task submissions, and a meta-analysis of system trends, successes, and failures.

Task Description
The MRQA 2019 Shared Task focuses on generalization to out-of-domain data. Participants trained models on a fixed training dataset containing examples from six QA datasets. We then evaluated their systems on examples from 12 heldout test datasets. For six of the test datasets, we provided participants with some development data; the other six datasets were entirely hidden-participants did not know the identity of these datasets.
We restricted the shared task to Englishlanguage extractive question answering: systems were given a question and context passage, and were asked to find a segment of text in the context that answers the question. This format is used by several commonly-used reading comprehension datasets, including SQuAD (Rajpurkar et al., 2016) and TriviaQA (Joshi et al., 2017). We found that the extractive format is general enough that we could convert many other existing datasets into this format. The simplicity of this format allowed us to focus on out-of-domain generalization, instead of other important but orthogonal challenges. 2 The datasets we used in our shared task are given in Table 1. The datasets differ in the following ways: • Passage distribution: Context passages come from many different sources, including Wikipedia, news articles, Web snippets, and textbooks.
• Question distribution: Questions are of different styles (e.g., entity-centric, relational) and come from different sources, including crowdworkers, domain experts, and exam writers.
• Joint distribution: The relationship between the passage and question also varies. Some questions were written based on the passage, while other questions were written independently, with context passages retrieved afterwards. Some questions were constructed to require multi-hop reasoning on the passage.
Evaluation criteria Systems are evaluated using exact match score (EM) and word-level F1score (F1), as is common in extractive question answering tasks (Rajpurkar et al., 2016;Joshi et al., 2017;Yang et al., 2018). EM only gives credit for predictions that exactly match (one of) the gold answer(s), whereas F1 gives a partial credit for partial word overlap with the gold answer(s). We follow the SQuAD evaluation normalization rules and ignore articles and punctuation when computing EM and F1 scores. While more strict evaluation (Kwiatkowski et al., 2019) computes scores based on the token indexes of the provided context, we compute scores based on answer string match (i.e., the prediction doesn't need to come from exact same annotated span as long as the predicted answer string matches the annotated answer string). We rank systems based on their macroaveraged test F1 scores across the 12 test datasets.

Dataset Curation
The MRQA 2019 Shared Task dataset is comprised of many sub-domains, each collected from a separate dataset. The dataset splits and subdomains are detailed in Table 1. As part of the collection process, we adapted each dataset to conform to the following unified, extractive format: 1. The answer to each question must appear as a span of tokens in the passage.
2. Passages may span multiple paragraphs or documents, but they are concatenated and truncated to the first 800 tokens. This eases the computational requirements for processing large documents efficiently.
The first requirement is motivated by the following reasons: • Extractive settings are easier to evaluate with stable metrics than abstractive settings.
• Unanswerable questions are hard to synthesize reliably on datasets without them. We investigated using distant supervision to automatically generate unanswerable questions, but found it would introduce a significant amount of noise.
• It is easier to convert multiple-choice datasets to extractive datasets than converting extractive datasets to multiple-choice, as it is difficult to generate challenging alternative answer options.
• Many of popular benchmark datasets are already extractive (or have extractive portions).

Sub-domain Splits
We partition the 18 sub-domains in the MRQA dataset into three splits: Split I These sub-domains are available for model training and development, but are not included in evaluation.  Additionally, we balance the testing portions of Splits II and III by re-partitioning the original subdomain datasets so that we have 1,500 examples per sub-domain. We partition by context, so that no single context is shared across both development and testing portions of either Split II or Split III. 3

Common Preprocessing
Datasets may contain contexts that are comprised of multiple documents or paragraphs. We concatenate all documents and paragraphs together. We separate documents with a [DOC] token, insert [TLE] tokens before each document title (if pro-vided), and separate paragraphs within a document with a [PAR] token.
Many of the original datasets do not have labeled answer spans. For these datasets we provide all occurrences of the answer string in the context in the dataset. Additionally, several of the original datasets contain multiple-choice questions. For these datasets, we keep the correct answer if it is contained in the context, and discard the other options. We filter questions that depend on the specific options (e.g., questions of the form "which of the following..." or "examples of ... include"). Removing multiple-choice options might introduce ambiguity (e.g., if multiple correct answers appear in the context but not in the original options). For these datasets, we attempt to control for quality by manually verifying random examples.

Sub-domain Datasets
In this section we describe the datasets used as sub-domains for MRQA. We focus on the modifications made to convert each dataset to the unified MRQA format. Please see Table 1 as well as the associated dataset papers for more details on each sub-domain's properties.
SQuAD (Rajpurkar et al., 2016) We used the SQuAD (Stanford Question Answering Dataset) dataset as the basis for the shared task format. 4 Crowdworkers are shown paragraphs from Wikipedia and are asked to write questions with extractive answers.
NewsQA (Trischler et al., 2017) Two sets of crowdworkers ask and answer questions based on CNN news articles. The "questioners" see only the article's headline and summary while the "answerers" see the full article. We discard questions that have no answer or are flagged in the dataset to be without annotator agreement.
TriviaQA (Joshi et al., 2017) Question and answer pairs are sourced from trivia and quiz-league websites. We use the web version of TriviaQA, where the contexts are retrieved from the results of a Bing search query.
SearchQA (Dunn et al., 2017) Question and answer pairs are sourced from the Jeopardy! TV show. The contexts are composed of retrieved snippets from a Google search query.
HotpotQA (Yang et al., 2018) Crowdworkers are shown two entity-linked paragraphs from Wikipedia and are asked to write and answer questions that require multi-hop reasoning to solve. In the original setting, these paragraphs are mixed with additional distractor paragraphs to make inference harder. We do not include the distractor paragraphs in our setting.
Natural Questions (Kwiatkowski et al., 2019) Questions are collected from information-seeking queries to the Google search engine by real users under natural conditions. Answers to the questions are annotated in a retrieved Wikipedia page by crowdworkers. Two types of annotations are collected: 1) the HTML bounding box containing enough information to completely infer the answer to the question (Long Answer), and 2) the subspan or sub-spans within the bounding box that comprise the actual answer (Short Answer). We use only the examples that have short answers, and use the long answer as the context.
BioASQ (Tsatsaronis et al., 2015) BioASQ, a challenge on large-scale biomedical semantic indexing and question answering, contains question and answer pairs that are created by domain experts. They are then manually linked to multiple related science (PubMed) articles. We download the full abstract of each of the linked articles to use as individual contexts (e.g., a single question can be linked to multiple, independent articles to create multiple QA-context pairs). We discard abstracts that do not exactly contain the answer.
DROP (Dua et al., 2019) DROP (Discrete Reasoning Over the content of Paragraphs) examples were collected similarly to SQuAD, where crowdworkers are asked to create question-answer pairs from Wikipedia paragraphs. The questions focus on quantitative reasoning, and the original dataset contains non-extractive numeric answers as well as extractive text answers. We restrict ourselves to the set of questions that are extractive.
DuoRC (Saha et al., 2018) We use the Para-phraseRC split of the DuoRC dataset. In this setting, two different plot summaries of the same movie are collected-one from Wikipedia and the other from IMDb. Two different sets of crowdworkers ask and answer questions about the movie plot, where the "questioners" are shown only the Wikipedia page, and the "answerers" are shown only the IMDb page. We discard questions that are marked as unanswerable.
RACE (Lai et al., 2017) ReAding Comprehension Dataset From Examinations (RACE) is collected from English reading comprehension exams for middle and high school Chinese students. We use the high school split (which is more challenging) and also filter out the implicit "fill in the blank" style questions (which are unnatural for this task).
RelationExtraction (Levy et al., 2017) Given a slot-filling dataset, 5 relations among entities are systematically transformed into questionanswer pairs using templates. For example, the educated at(x, y) relationship between two entities x and y appearing in a sentence can be expressed as "Where was x educated at?" with answer y. Multiple templates for each type of relation are collected. We use the dataset's zeroshot benchmark split (generalization to unseen relations), and only keep the positive examples.
TextbookQA (Kembhavi et al., 2017) Text-bookQA is collected from lessons from middle school Life Science, Earth Science, and Physical Science textbooks. We do not include questions that are accompanied with a diagram, or that are "True or False" questions.
BioProcess (Berant et al., 2014) Paragraphs are sourced from a biology textbook, and question and answer pairs about those paragraphs are then created by domain experts.
ComplexWebQ (Talmor and Berant, 2018) ComplexWebQuestions is collected by crowdworkers who are shown compositional, formal queries against Freebase, and are asked to rephrase them in natural language. Thus, by design, questions require multi-hop reasoning. For the context, we use the default web snippets provided by the authors. We use only single-answer questions of type "composition" or "conjunction".
MCTest (Richardson et al., 2013) Passages accompanied with questions and answers are written by crowdworkers. The passages are fictional, elementary-level, children's stories.
QAMR (Michael et al., 2018) To construct the Question-Answer Meaning Representation (QAMR) dataset, crowdworkers are presented with an English sentence along with target nonstopwords from the sentence. They are then asked to create as many question-answer pairs as possible that contain at least one of the target words (and for which the answer is a span of the sentence). These questions combine to cover most of the predicate-argument structures present. We use only the filtered 6 subset of the Wikipedia portion of the dataset.
QAST (Lamel et al., 2008) We use Task 1 of the Question Answering on Speech Transcriptions (QAST) dataset, where contexts are taken from manual transcripts of spoken lectures on "speech and language processing." Questions about named entities found in the transcriptions are created by English native speakers. Each lecture contains around 1 hour of transcribed text. To reduce the length to meet our second requirement (≤ 800 tokens), for each question we manually selected a sub-section of the lecture that contained the answer span, as well as sufficient surrounding context to answer it.
TREC (Baudiš andŠedivý, 2015) The Text REtrieval Conference (TREC) dataset is curated 6 The questions that are valid and non-redundant. from the TREC QA tasks (Voorhees and Tice, 2000) from 1999-2002. The questions are factoid. Accompanying passages are supplied using the Document Retriever from , if the answer is found within the first 800 tokens of any of the top 5 retrieved Wikipedia documents (we take the highest ranked document if multiple documents meet this requirement).

Baseline Model
We implemented a simple, multi-task baseline model based on BERT (Devlin et al., 2018), following the MultiQA model (Talmor and Berant, 2019). Our method works as follows: Modeling Given a question q consisting of m tokens {q 1 , . . . , q m } and a passage p of n tokens {p 1 , . . . , p n }, we first concatenate q and p with special tokens to obtain a joint context {[CLS], q 1 , . . . , q m , [SEP], p 1 , . . . , p n , [SEP]}. We then encode the joint context with BERT to obtain contextualized passage representations {h 1 , . . . , h n }.
We train separate MLPs to predict start and end indices independently, and decode the final span using arg max i,j {p start (i) × p end (j)}.
Preprocessing Following Devlin et al. (2018), we create p and q by tokenizing every example using a vocabulary of 30,522 word pieces. As BERT accepts a maximum sequence length of 512, we generate multiple chunks {p (1) , . . . , p (k) } per example by sliding a 512 token window (of the joint context, including q) over the entire length of the original passage, with a stride of 128 tokens.
Training During training we select only the chunks that contain answers. We maximize the log-likelihood of the first occurrence of the gold answer in each of these chunks, and backpropagate into BERT's parameters (and the MLP parameters). At test time we output the span with the maximal logit across all chunks.

Multi-task Training
We sample up to 75K examples from each training dataset, combine them, and create mixed batches of examples from all of the data. We then follow the same training procedure as before on all the composed training dataset batches.

Shared Task Submissions
Our shared task lasted for 3 months from May to August in 2019. All submissions were handled through the CodaLab platform. 7 In total, we received submissions from 10 different teams for the final evaluation (Table 2). Of these, 6 teams submitted their system description paper. We will describe each of them briefly below.

D-Net (Li et al., 2019)
The submission from Baidu adopts multiple pre-trained language models (LMs), including BERT (Devlin et al., 2018), XLNet (Yang et al., 2019), and ERNIE 2.0 . Unlike other submissions which use only one pretrained LM, they experiment with 1) training LMs with extra raw text data drawn from science questions and search snippets domains, and 2) multitasking with auxiliary tasks such as natural language inference and paragraph ranking (Williams et al., 2017). Ultimately, however, the final system is an ensemble of an XLNet-based model and an ERNIE-based model, without auxiliary multitask or augmented LM training.

Delphi (Longpre et al., 2019)
The submission from Apple investigates the effects of pre-trained language models (BERT vs XLNet), various data sampling strategies, and data augmentation techniques via back-translation. Their final submission uses XLNet (Yang et al., 2019) as the base model, with carefully sampled training instances from negative examples (hence augmenting the model with a no-answer option) and the six training datasets. The final submission does not include data augmentation, as it did not improve performance during development.

HLTC (Su et al., 2019)
The submission from HKUST studies different data-feeding schemes, namely shuffling instances from all datasets versus shuffling dataset-ordering only. Their submission is built on top of XL-Net, with a multilayer perceptron layer for span prediction. They also attempted to substitute the MLP layer with a more complex attention-overattention (AoA) (Cui et al., 2017) layer on top of XLNet, but did not find it to be helpful. (Takahashi et al., 2019) The submission from Fuji Xerox adds a mixtureof-experts (MoE) (Jacobs et al., 1991) layer on top of a BERT-based architecture. They also use a multi-task learning framework trained together with natural language inference (NLI) tasks. Their final submission is an ensemble of three models trained with different random seeds.

CLER
5.5 Adv. Train (Lee et al., 2019) The submission from 42Maru and Samsung Research proposes an adversarial training framework, where a domain discriminator predicts the underlying domain label from the QA model's hidden representations, while the QA model tries to learn to arrange its hidden representations such that the discriminator is thwarted. Through this process, they aim to learn domain (dataset) invariant features that can generalize to unseen domains. The submission is built based on the provided BERT baselines.

HierAtt (Osama et al., 2019)
The submission from Alexandria University uses the BERT-Base model to provide feature representations. Unlike other models which allowed finetuning of the language model parameters during training, this submission only trains model parameters associated with the question answering task, while keeping language model parameters frozen. The model consists of two attention mechanisms: one bidirectional attention layer used to model the interaction between the passage and the question, and one self-attention layer applied to both the question and the passage. Table 3 lists the macro-averaged F1 scores of all the submissions on both the development and testing portions of the MRQA dataset. The teams are ranked by the F1 scores on the hidden testing portions of the 12 datasets (Split II and III in Section 3.1). As seen in Table 3, many of the submissions outperform our BERT-Large baseline significantly. The best-performing system, D-Net (Li et al., 2019), achieves an F1 score of 72.5, which is a 10.7 point absolute improvement over our baseline, and 11.5 and 10.0 point improvements, respectively, on Split II (with the development por-

FT XLNet
Harbin Institute of Technology HLTC (Su et al., 2019) Hong Kong University of Science & Technology BERT-cased-whole-word Aristo @ AI2 CLER (Takahashi et al., 2019) Fuji Xerox Co., Ltd. Adv. Train (Lee et al., 2019) 42Maru and Samsung Research BERT-Multi-Finetune Beijing Language and Culture University PAL IN DOMAIN University of California Irvine HierAtt (Osama et al., 2019) Alexandria University   Table 4: Macro-averaged F1 scores based on the dataset characteristics as defined in Table 1. Best denotes the best shared task result and Base denotes our BERT-Large baseline.
tions provided) and Split III datasets (completely hidden to the participants).
We evaluate all the submissions on the indomain datasets (Split I) in Table 3 and find that there is a very strong correlation between indomain and out-of-domain performance. The top submissions on the out-of-domain datasets also obtain the highest scores on the six datasets that we provided for training.
We present per-dataset performances for 12 evaluation datasets in the appendix. Across the board, many submitted systems greatly outperform our baselines. Among the 12 datasets, performance on the DROP dataset has improved the most-from 43.5 F1 to 61.5 F1-while performance on the RelationExtraction dataset has improved the least (84.9 F1 vs. 89.0 F1). The models with higher average scores seemed to outperform in most datasets: the performance rankings of submissions are mostly preserved on individual datasets.

Summary of Findings
Improvements per data types We analyzed the average performance across the various types of datasets that are represented in Table 1. Table 4 summarizes our observations: (1) the datasets with naturally collected questions (either crowdsourced or curated by domain experts) all obtain large improvements; (2) The datasets collected from Wikipedia or education materials (textbooks and Science articles) receive bigger gains compared to those collected from Web snippets or transcriptions; and (3) There is a bigger improvement for datasets in which questions are posed dependent on the passages compared to those with independently collected questions (11.9 vs. 9.2 points).
Pre-trained language models The choice of pre-trained language model has a significant impact on the QA performance, as well as the generalization ability. Table 5 summarizes the pretrained models each submission is based on, along with its evaluation F1 score. The top three performing systems all use XLNet instead of BERT-Large-this isolated change in pre-trained language model alone yields a significant gain in overall in-and out-of-domain performance. Li et al. (2019) argues that XLNet shows superior performances on datasets with discrete reasoning, such as DROP and RACE. Su et al. (2019), however, also use XLNet, but does not show strong gains on the DROP or RACE datasets.
The winning system ensembled two different pre-trained language models. Only one other submission (Takahashi et al., 2019) used an ensemble for their final submission, merging the same LM with different random seeds.  Table 5: Pretrained language models used in the shared task submissions. *-L and *-B denote large and base versions of the models. Data sampling Our shared task required all participants to use our provided training data, compiled from six question answering datasets, and disallowed the use of any other questionanswering data for training. Within these restrictions, we encouraged participants to explore how to best utilize the provided data. Inspired by Talmor and Berant (2019), two submissions (Su et al., 2019;Longpre et al., 2019) analyzed similarities between datasets. Unsurprisingly, the performance improved significantly when fine-tuned on the training dataset most similar to the evaluation dataset of interest. Su et al. (2019) found each of the development (Split II) datasets resembles one or two training datasets (Split I)-and thus training with all datasets is crucial for generalization across the  multiple domains. They experimented with datafeeding methodologies, and found that shuffling instances of all six training datasets is more effective than sequentially feeding all examples from each dataset, one dataset after another. Additionally, Longpre et al. (2019) observed that the models fine-tuned on SearchQA and Triv-iaQA achieve relatively poor results across all the evaluation sets (they are both trivia-based, distantly supervised, and long-context datasets). Downsampling examples from these datasets increases the overall performance. They also found that, although our shared task focuses on answerable questions, sampling negative examples leads to significant improvements (up to +1.5 F1 on Split II and up to +4 F1 on Split I). Since most systems follow our baseline model (Section 4) by doing inference over chunks of tokens, not all examples fed to these models are actually guaranteed to contain an answer span.
Multi-task learning Two submissions attempted to learn the question answering model together with other auxiliary tasks, namely natural language inference (Takahashi et al., 2019;Li et al., 2019) or paragraph ranking (Li et al., 2019) (i.e., classifying whether given passages contains an answer to the question or not). This could improve the generalization performance on question answering for two reasons. First, the additional training simply exposes the model to more diverse domains, as the entailment dataset (Williams et al., 2017) contains multiple domains ranging from fiction to telephone conversations. Second, reasoning about textual entailment is often necessary for question answering, while passage ranking (or classification) is an easier version of extractive question answering, where the model has to identify the passage containing the answer instead of exact span.
Both systems introduced task-specific fully connected layers while sharing lower level representations across different tasks. While Takahashi et al. (2019) showed a modest gain by multi-tasking with NLI tasks (+0.7 F1 score on the development portion of Split II), Li et al. (2019) reported that multitasking did not improve the performance of their best model. Adversarial Training One submission (Lee et al., 2019) introduced an adversarial training framework for generalization. The goal is to learn domain-invariant features (i.e., features that can generalize to unseen test domains) by jointly training with a domain discriminator, which predicts the dataset (domain) for each example. According to Lee et al. (2019), this adversarial training helped on most of the datasets (9 out of 12), but also hurt performance on some of them. It finally led to +1.9 F1 gain over their BERT-Base baseline, although the gain was smaller (+0.4 F1) for their stronger BERT-Large baseline.
Ensembles Most extractive QA models, which output a logit for the start index and another for the end index, can be ensembled by adding the start and end logits from models trained with different random seeds. This has shown to improve performances across many model classes, as can be seen from most dataset leaderboards. The results from the shared task also show similar trends. A few submissions (Takahashi et al., 2019;Li et al., 2019) tried ensembling, and all reported modest gains. While ensembling is a quick recipe for a small gain in performance, it also comes at the cost of computational efficiency-both at training and at inference time.
Related to ensembling, Takahashi et al. (2019) uses a mixture of experts (Jacobs et al., 1991) layer, which learns a gating function to ensemble different weights, adaptively based on the input.

Comparison to In-domain Fine-tuning
Lastly, we report how the best shared task performance compares to in-domain fine-tuning performance of our baseline. Section 6.1 shows large improvements by the top shared task model, D-Net, over our baseline. We analyze to what extent the reduced performance on out-of-domain datasets can be overcome by exposing the baseline to only a few samples from the target distributions. As suggested by , if the model can generalize with a few examples from the new domain, poor performance on that domain is an indicator of a lack of training data diversity, rather than of fundamental model generalization weaknesses. Figure 1 presents our results on the six datasets from Split II, where we have individually finetuned the BERT-Large baseline on each of the Split II dev datasets and tested on the Split II test datasets. We see that while the gap to D-Net shrinks on all datasets (overall performance increases by 4.6 F1), surprisingly it is only completely bridged in one of the settings (RelationExtraction). This is potentially because this dataset covers only a limited number of relations, so having in-domain data helps significantly. This suggests that D-Net (and the others close to it in performance) is an overall stronger model-a conclusion also supported by its gain on in-domain data (Split I).

Conclusions
We have presented the MRQA 2019 Shared Task, which focused on testing whether reading comprehension systems can generalize to examples outside of their training domain. Many submissions improved significantly over our baseline, and investigated a wide range of techniques.
Going forward, we believe it will become increasingly important to build NLP systems that generalize across domains. As NLP models be-come more widely deployed, they must be able to handle diverse inputs, many of which may differ from those seen during training. By running this shared task and releasing our shared task datasets, we hope to shed more light how to build NLP systems that generalize beyond their training distribution.