prompt_front: You are given a scientific article and a question. Answer the question as concisely as you can, using a single phrase or sentence if possible. If the question cannot be answered based on the information in the article, write "unanswerable". If the question is a yes/no question, answer "yes", "no", or "unanswerable". Do not provide any explanation.

Please answer the questions based on the text provided below. What were their distribution results?
Context 1: 10pt
1 10pt
[ Characterizing Political Fake News in Twitter by its Meta-DataJulio Amador Díaz LópezAxel Oehmichen Miguel Molina-Solana( j amador, axelfrancois oehmichen11, mmolinas@imperial ac uk ) Imperial College London This article presents a preliminary approach towards characterizing political fake news on Twitter through the analysis of their meta-data  In particular, we focus on more than 1 5M tweets collected on the day of the election of Donald Trump as 45th president of the United States of America  We use the meta-data embedded within those tweets in order to look for differences between tweets containing fake news and tweets not containing them  Specifically, we perform our analysis only on tweets that went viral, by studying proxies for users' exposure to the tweets, by characterizing accounts spreading fake news, and by looking at their polarization  We found significant differences on the distribution of followers, the number of URLs on tweets, and the verification of the users 
]
Introduction
While fake news, understood as deliberately misleading pieces of information, have existed since long ago (e g  it is not unusual to receive news falsely claiming the death of a celebrity), the term reached the mainstream, particularly so in politics, during the 2016 presidential election in the United States BIBREF0   Since then, governments and corporations alike (e g  Google BIBREF1 and Facebook BIBREF2 ) have begun efforts to tackle fake news as they can affect political decisions BIBREF3   Yet, the ability to define, identify and stop fake news from spreading is limited 
Since the Obama campaign in 2008, social media has been pervasive in the political arena in the United States  Studies report that up to 62% of American adults receive their news from social media BIBREF4   The wide use of platforms such as Twitter and Facebook has facilitated the diffusion of fake news by simplifying the process of receiving content with no significant third party filtering, fact-checking or editorial judgement  Such characteristics make these platforms suitable means for sharing news that, disguised as legit ones, try to confuse readers 
Such use and their prominent rise has been confirmed by Craig Silverman, a Canadian journalist who is a prominent figure on fake news BIBREF5 : “In the final three months of the US presidential campaign, the top-performing fake election news stories on Facebook generated more engagement than the top stories from major news outlet” 
Our current research hence departs from the assumption that social media is a conduit for fake news and asks the question of whether fake news (as spam was some years ago) can be identified, modelled and eventually blocked  In order to do so, we use a sample of more that 1 5M tweets collected on November 8th 2016 —election day in the United States— with the goal of identifying features that tweets containing fake news are likely to have  As such, our paper aims to provide a preliminary characterization of fake news in Twitter by looking into meta-data embedded in tweets  Considering meta-data as a relevant factor of analysis is in line with findings reported by Morris et al  BIBREF6   We argue that understanding differences between tweets containing fake news and regular tweets will allow researchers to design mechanisms to block fake news in Twitter 
Specifically, our goals are: 1) compare the characteristics of tweets labelled as containing fake news to tweets labelled as not containing them, 2) characterize, through their meta-data, viral tweets containing fake news and the accounts from which they originated, and 3) determine the extent to which tweets containing fake news expressed polarized political views 
For our study, we used the number of retweets to single-out those that went viral within our sample  Tweets within that subset (viral tweets hereafter) are varied and relate to different topics  We consider that a tweet contains fake news if its text falls within any of the following categories described by Rubin et al  BIBREF7 (see next section for the details of such categories): serious fabrication, large-scale hoaxes, jokes taken at face value, slanted reporting of real facts and stories where the truth is contentious  The dataset BIBREF8 , manually labelled by an expert, has been publicly released and is available to researchers and interested parties 
From our results, the following main observations can be made:
Our findings resonate with similar work done on fake news such as the one from Allcot and Gentzkow BIBREF9   Therefore, even if our study is a preliminary attempt at characterizing fake news on Twitter using only their meta-data, our results provide external validity to previous research  Moreover, our work not only stresses the importance of using meta-data, but also underscores which parameters may be useful to identify fake news on Twitter 
The rest of the paper is organized as follows  The next section briefly discusses where this work is located within the literature on fake news and contextualizes the type of fake news we are studying  Then, we present our hypotheses, the data, and the methodology we follow  Finally, we present our findings, conclusions of this study, and future lines of work 
Defining Fake news
Our research is connected to different strands of academic knowledge related to the phenomenon of fake news  In relation to Computer Science, a recent survey by Conroy and colleagues BIBREF10 identifies two popular approaches to single-out fake news  On the one hand, the authors pointed to linguistic approaches consisting in using text, its linguistic characteristics and machine learning techniques to automatically flag fake news  On the other, these researchers underscored the use of network approaches, which make use of network characteristics and meta-data, to identify fake news 
With respect to social sciences, efforts from psychology, political science and sociology, have been dedicated to understand why people consume and/or believe misinformation BIBREF11 , BIBREF12 , BIBREF13 , BIBREF14   Most of these studies consistently reported that psychological biases such as priming effects and confirmation bias play an important role in people ability to discern misinformation 
In relation to the production and distribution of fake news, a recent paper in the field of Economics BIBREF9 found that most fake news sites use names that resemble those of legitimate organizations, and that sites supplying fake news tend to be short-lived  These authors also noticed that fake news items are more likely shared than legitimate articles coming from trusted sources, and they tend to exhibit a larger level of polarization 
The conceptual issue of how to define fake news is a serious and unresolved issue  As the focus of our work is not attempting to offer light on this, we will rely on work by other authors to describe what we consider as fake news  In particular, we use the categorization provided by Rubin et al  BIBREF7   The five categories they described, together with illustrative examples from our dataset, are as follows:
Research Hypotheses
Previous works on the area (presented in the section above) suggest that there may be important determinants for the adoption and diffusion of fake news  Our hypotheses builds on them and identifies three important dimensions that may help distinguishing fake news from legit information:
Taking those three dimensions into account, we propose the following hypotheses about the features that we believe can help to identify tweets containing fake news from those not containing them  They will be later tested over our collected dataset 
Exposure 
Characterization 
Polarization 
Data and Methodology
For this study, we collected publicly available tweets using Twitter's public API  Given the nature of the data, it is important to emphasize that such tweets are subject to Twitter's terms and conditions which indicate that users consent to the collection, transfer, manipulation, storage, and disclosure of data  Therefore, we do not expect ethical, legal, or social implications from the usage of the tweets  Our data was collected using search terms related to the presidential election held in the United States on November 8th 2016  Particularly, we queried Twitter's streaming API, more precisely the filter endpoint of the streaming API, using the following hashtags and user handles: #MyVote2016, #ElectionDay, #electionnight, @realDonaldTrump and @HillaryClinton  The data collection ran for just one day (Nov 8th 2016) 
One straightforward way of sharing information on Twitter is by using the retweet functionality, which enables a user to share a exact copy of a tweet with his followers  Among the reasons for retweeting, Body et al  BIBREF15 reported the will to: 1) spread tweets to a new audience, 2) to show one’s role as a listener, and 3) to agree with someone or validate the thoughts of others  As indicated, our initial interest is to characterize tweets containing fake news that went viral (as they are the most harmful ones, as they reach a wider audience), and understand how it differs from other viral tweets (that do not contain fake news)  For our study, we consider that a tweet went viral if it was retweeted more than 1000 times 
Once we have the dataset of viral tweets, we eliminated duplicates (some of the tweets were collected several times because they had several handles) and an expert manually inspected the text field within the tweets to label them as containing fake news, or not containing them (according to the characterization presented before)  This annotated dataset BIBREF8 is publicly available and can be freely reused 
Finally, we use the following fields within tweets (from the ones returned by Twitter's API) to compare their distributions and look for differences between viral tweets containing fake news and viral tweets not containing fake news:
In the following section, we provide graphical descriptions of the distribution of each of the identified attributes for the two sets of tweets (those labelled as containing fake news and those labelled as not containing them)  Where appropriate, we normalized and/or took logarithms of the data for better representation  To gain a better understanding of the significance of those differences, we use the Kolmogorov-Smirnov test with the null hypothesis that both distributions are equal 
Results
The sample collected consisted on 1 785 855 tweets published by 848 196 different users  Within our sample, we identified 1327 tweets that went viral (retweeted more than 1000 times by the 8th of November 2016) produced by 643 users  Such small subset of viral tweets were retweeted on 290 841 occasions in the observed time-window 
The 1327 `viral' tweets were manually annotated as containing fake news or not  The annotation was carried out by a single person in order to obtain a consistent annotation throughout the dataset  Out of those 1327 tweets, we identified 136 as potentially containing fake news (according to the categories previously described), and the rest were classified as `non containing fake news'  Note that the categorization is far from being perfect given the ambiguity of fake news themselves and human judgement involved in the process of categorization  Because of this, we do not claim that this dataset can be considered a ground truth 
The following results detail characteristics of these tweets along the previously mentioned dimensions  Table TABREF23 reports the actual differences (together with their associated p-values) of the distributions of viral tweets containing fake news and viral tweets not containing them for every variable considered 
Exposure
Figure FIGREF24 shows that, in contrast to other kinds of viral tweets, those containing fake news were created more recently  As such, Twitter users were exposed to fake news related to the election for a shorter period of time 
However, in terms of retweets, Figure FIGREF25 shows no apparent difference between containing fake news or not containing them  That is confirmed by the Kolmogorov-Smirnoff test, which does not discard the hypothesis that the associated distributions are equal 
In relation to the number of favourites, users that generated at least a viral tweet containing fake news appear to have, on average, less favourites than users that do not generate them  Figure FIGREF26 shows the distribution of favourites  Despite the apparent visual differences, the difference are not statistically significant 
Finally, the number of hashtags used in viral fake news appears to be larger than those in other viral tweets  Figure FIGREF27 shows the density distribution of the number of hashtags used  However, once again, we were not able to find any statistical difference between the average number of hashtags in a viral tweet and the average number of hashtags in viral fake news 
Characterization
We found that 82 users within our sample were spreading fake news (i e  they produced at least one tweet which was labelled as fake news)  Out of those, 34 had verified accounts, and the rest were unverified  From the 48 unverified accounts, 6 have been suspended by Twitter at the date of writing, 3 tried to imitate legitimate accounts of others, and 4 accounts have been already deleted  Figure FIGREF28 shows the proportion of verified accounts to unverified accounts for viral tweets (containing fake news vs  not containing fake news)  From the chart, it is clear that there is a higher chance of fake news coming from unverified accounts 
Turning to friends, accounts distributing fake news appear to have, on average, the same number of friends than those distributing tweets with no fake news  However, the density distribution of friends from the accounts (Figure FIGREF29 ) shows that there is indeed a statistically significant difference in their distributions 
If we take into consideration the number of followers, accounts generating viral tweets with fake news do have a very different distribution on this dimension, compared to those accounts generating viral tweets with no fake news (see Figure FIGREF30 )  In fact, such differences are statistically significant 
A useful representation for friends and followers is the ratio between friends/followers  Figures FIGREF31 and FIGREF32 show this representation  Notice that accounts spreading viral tweets with fake news have, on average, a larger ratio of friends/followers  The distribution of those accounts not generating fake news is more evenly distributed 
With respect to the number of mentions, Figure FIGREF33 shows that viral tweets labelled as containing fake news appear to use mentions to other users less frequently than viral tweets not containing fake news  In other words, tweets containing fake news mostly contain 1 mention, whereas other tweets tend to have two)  Such differences are statistically significant 
The analysis (Figure FIGREF34 ) of the presence of media in the tweets in our dataset shows that tweets labelled as not containing fake news appear to present more media elements than those labelled as fake news  However, the difference is not statistically significant 
On the other hand, Figure FIGREF35 shows that viral tweets containing fake news appear to include more URLs to other sites than viral tweets that do not contain fake news  In fact, the difference between the two distributions is statistically significant (assuming INLINEFORM0 ) 
Polarization
Finally, manual inspection of the text field of those viral tweets labelled as containing fake news shows that 117 of such tweets expressed support for Donald Trump, while only 8 supported Hillary Clinton  The remaining tweets contained fake news related to other topics, not expressing support for any of the candidates 
Discussion
As a summary, and constrained by our existing dataset, we made the following observations regarding differences between viral tweets labelled as containing fake news and viral tweets labelled as not containing them:
These findings (related to our initial hypothesis in Table TABREF44 ) clearly suggest that there are specific pieces of meta-data about tweets that may allow the identification of fake news  One such parameter is the time of exposure  Viral tweets containing fake news are shorter-lived than those containing other type of content  This notion seems to resonate with our findings showing that a number of accounts spreading fake news have already been deleted or suspended by Twitter by the time of writing  If one considers that researchers using different data have found similar results BIBREF9 , it appears that the lifetime of accounts, together with the age of the questioned viral content could be useful to identify fake news  In the light of this finding, accounts newly created should probably put under higher scrutiny than older ones  This in fact, would be a nice a-priori bias for a Bayesian classifier 
Accounts spreading fake news appear to have a larger proportion of friends/followers (i e  they have, on average, the same number of friends but a smaller number of followers) than those spreading viral content only  Together with the fact that, on average, tweets containing fake news have more URLs than those spreading viral content, it is possible to hypothesize that, both, the ratio of friends/followers of the account producing a viral tweet and number of URLs contained in such a tweet could be useful to single-out fake news in Twitter  Not only that, but our finding related to the number of URLs is in line with intuitions behind the incentives to create fake news commonly found in the literature BIBREF9 (in particular that of obtaining revenue through click-through advertising) 
Finally, it is interesting to notice that the content of viral fake news was highly polarized  This finding is also in line with those of Alcott et al  BIBREF9   This feature suggests that textual sentiment analysis of the content of tweets (as most researchers do), together with the above mentioned parameters from meta-data, may prove useful for identifying fake news 
Conclusions
With the election of Donald Trump as President of the United States, the concept of fake news has become a broadly-known phenomenon that is getting tremendous attention from governments and media companies  We have presented a preliminary study on the meta-data of a publicly available dataset of tweets that became viral during the day of the 2016 US presidential election  Our aim is to advance the understanding of which features might be characteristic of viral tweets containing fake news in comparison with viral tweets without fake news 
We believe that the only way to automatically identify those deceitful tweets (i e  containing fake news) is by actually understanding and modelling them  Only then, the automation of the processes of tagging and blocking these tweets can be successfully performed  In the same way that spam was fought, we anticipate fake news will suffer a similar evolution, with social platforms implementing tools to deal with them  With most works so far focusing on the actual content of the tweets, ours is a novel attempt from a different, but also complementary, angle 
Within the used dataset, we found there are differences around exposure, characteristics of accounts spreading fake news and the tone of the content  Those findings suggest that it is indeed possible to model and automatically detect fake news  We plan to replicate and validate our experiments in an extended sample of tweets (until 4 months after the US election), and tests the predictive power of the features we found relevant within our sample 
Author Disclosure Statement
No competing financial interest exist 

ground_truth: Distributions of Followers, Friends and URLs are significantly different between the set of tweets containing fake news and those non containing them, but for Favourites, Mentions, Media, Retweets and Hashtags they are not significantly different

prompt_front: You are given a scientific article and a question. Answer the question as concisely as you can, using a single phrase or sentence if possible. If the question cannot be answered based on the information in the article, write "unanswerable". If the question is a yes/no question, answer "yes", "no", or "unanswerable". Do not provide any explanation.

Please answer the questions based on the text provided below. Is it possible to convert a cloze-style questions to a naturally-looking questions?
Context 2: Introduction
Deep learning systems have shown a lot of promise for extractive Question Answering (QA), with performance comparable to humans when large scale data is available  However, practitioners looking to build QA systems for specific applications may not have the resources to collect tens of thousands of questions on corpora of their choice  At the same time, state-of-the-art machine reading systems do not lend well to low-resource QA settings where the number of labeled question-answer pairs are limited (c f  Table 2 )  Semi-supervised QA methods like BIBREF0 aim to improve this performance by leveraging unlabeled data which is easier to collect 
In this work, we present a semi-supervised QA system which requires the end user to specify a set of base documents and only a small set of question-answer pairs over a subset of these documents  Our proposed system consists of three stages  First, we construct cloze-style questions (predicting missing spans of text) from the unlabeled corpus; next, we use the generated clozes to pre-train a powerful neural network model for extractive QA BIBREF1 , BIBREF2 ; and finally, we fine-tune the model on the small set of provided QA pairs 
Our cloze construction process builds on a typical writing phenomenon and document structure: an introduction precedes and summarizes the main body of the article  Many large corpora follow such a structure, including Wikipedia, academic papers, and news articles  We hypothesize that we can benefit from the un-annotated corpora to better answer various questions – at least ones that are lexically similar to the content in base documents and directly require factual information 
We apply the proposed system on three datasets from different domains – SQuAD BIBREF3 , TriviaQA-Web BIBREF4 and the BioASQ challenge BIBREF5   We observe significant improvements in a low-resource setting across all three datasets  For SQuAD and TriviaQA, we attain an F1 score of more than 50% by merely using 1% of the training data  Our system outperforms the approaches for semi-supervised QA presented in BIBREF0 , and a baseline which uses the same unlabeled data but with a language modeling objective for pretraining  In the BioASQ challenge, we outperform the best performing system from previous year's challenge, improving over a baseline which does transfer learning from the SQuAD dataset  Our analysis reveals that questions which ask for factual information and match to specific parts of the context documents benefit the most from pretraining on automatically constructed clozes 
Related Work
Semi-supervised learning augments the labeled dataset $L$ with a potentially larger unlabeled dataset $U$   BIBREF0 presented a model, GDAN, which trained an auxiliary neural network to generate questions from passages by reinforcement learning, and augment the labeled dataset with the generated questions to train the QA model  Here we use a much simpler heuristic to generate the auxiliary questions, which also turns out to be more effective as we show superior performance compared to GDAN  Several approaches have been suggested for generating natural questions BIBREF6 , BIBREF7 , BIBREF8 , however none of them show a significant improvement of using the generated questions in a semi-supervised setting  Recent papers also use unlabeled data for QA by training large language models and extracting contextual word vectors from them to input to the QA model BIBREF9 , BIBREF10 , BIBREF11   The applicability of this method in the low-resource setting is unclear as the extra inputs increase the number of parameters in the QA model, however, our pretraining can be easily applied to these models as well 
Domain adaptation (and Transfer learning) leverage existing large scale datasets from a source domain (or task) to improve performance on a target domain (or task)  For deep learning and QA, a common approach is to pretrain on the source dataset and then fine-tune on the target dataset BIBREF12 , BIBREF13   BIBREF14 used SQuAD as a source for the target BioASQ dataset, and BIBREF15 used Book Test BIBREF16 as source for the target SQuAD dataset  BIBREF17 transfer learned model layers from the tasks of sequence labeling, text classification and relation classification to show small improvements on SQuAD  All these works use manually curated source datatset, which in themselves are expensive to collect  Instead, we show that it is possible to automatically construct the source dataset from the same domain as the target, which turns out to be more beneficial in terms of performance as well (c f  Section "Experiments & Results" )  Several cloze datasets have been proposed in the literature which use heuristics for construction BIBREF18 , BIBREF19 , BIBREF20   We further see the usability of such a dataset in a semi-supervised setting 
Methodology
Our system comprises of following three steps:
Cloze generation: Most of the documents typically follow a template, they begin with an introduction that provides an overview and a brief summary for what is to follow  We assume such a structure while constructing our cloze style questions  When there is no clear demarcation, we treat the first $K\%$ (hyperparameter, in our case 20%) of the document as the introduction  While noisy, this heuristic generates a large number of clozes given any corpus, which we found to be beneficial for semi-supervised learning despite the noise 
We use a standard NLP pipeline based on Stanford CoreNLP (for SQuAD, TrivaQA and PubMed) and the BANNER Named Entity Recognizer (only for PubMed articles) to identify entities and phrases  Assume that a document comprises of introduction sentences $\lbrace q_1, q_2,     q_n\rbrace $ , and the remaining passages $\lbrace p_1, p_2,    p_m\rbrace $   Additionally, let's say that each sentence $q_i$ in introduction is composed of words $\lbrace w_1, w_2,     w_{l_{q_i}}\rbrace $ , where $l_{q_i}$ is the length of $q_i$   We consider a $\text{match} (q_i, p_j)$ , if there is an exact string match of a sequence of words $\lbrace w_k, w_{k+1},    w_{l_{q_i}}\rbrace $ between the sentence $q_i$ and passage $p_j$   If this sequence is either a noun phrase, verb phrase, adjective phrase or a named entity in $\lbrace p_1, p_2,    p_m\rbrace $0 , as recognized by CoreNLP or BANNER, we select it as an answer span $\lbrace p_1, p_2,    p_m\rbrace $1   Additionally, we use $\lbrace p_1, p_2,    p_m\rbrace $2 as the passage $\lbrace p_1, p_2,    p_m\rbrace $3 and form a cloze question $\lbrace p_1, p_2,    p_m\rbrace $4 from the answer bearing sentence $\lbrace p_1, p_2,    p_m\rbrace $5 by replacing $\lbrace p_1, p_2,    p_m\rbrace $6 with a placeholder  As a result, we obtain passage-question-answer ( $\lbrace p_1, p_2,    p_m\rbrace $7 ) triples (Table 1 shows an example)  As a post-processing step, we prune out $\lbrace p_1, p_2,    p_m\rbrace $8 triples where the word overlap between the question (Q) and passage (P) is less than 2 words (after excluding the stop words) 
The process relies on the fact that answer candidates from the introduction are likely to be discussed in detail in the remainder of the article  In effect, the cloze question from the introduction and the matching paragraph in the body forms a question and context passage pair  We create two cloze datasets, one each from Wikipedia corpus (for SQuAD and TriviaQA) and PUBMed academic papers (for the BioASQ challenge), consisting of 2 2M and 1M clozes respectively  From analyzing the cloze data manually, we were able to answer 76% times for the Wikipedia set and 80% times for the PUBMed set using the information in the passage  In most cases the cloze paraphrased the information in the passage, which we hypothesized to be a useful signal for the downstream QA task 
We also investigate the utility of forming subsets of the large cloze corpus, where we select the top passage-question-answer triples, based on the different criteria, like i) jaccard similarity of answer bearing sentence in introduction and the passage ii) the tf-idf scores of answer candidates and iii) the length of answer candidates  However, we empirically find that we were better off using the entire set rather than these subsets 
Pre-training: We make use of the generated cloze dataset to pre-train an expressive neural network designed for the task of reading comprehension  We work with two publicly available neural network models – the GA Reader BIBREF2 (to enable comparison with prior work) and BiDAF + Self-Attention (SA) model from BIBREF1 (which is among the best performing models on SQuAD and TriviaQA)  After pretraining, the performance of BiDAF+SA on a dev set of the (Wikipedia) cloze questions is 0 58 F1 score and 0 55 Exact Match (EM) score  This implies that the cloze corpus is neither too easy, nor too difficult to answer 
Fine Tuning: We fine tune the pre-trained model, from the previous step, over a small set of labelled question-answer pairs  As we shall later see, this step is crucial, and it only requires a handful of labelled questions to achieve a significant proportion of the performance typically attained by training on tens of thousands of questions 
Datasets
We apply our system to three datasets from different domains  SQuAD BIBREF3 consists of questions whose answers are free form spans of text from passages in Wikipedia articles  We follow the same setting as in BIBREF0 , and split $10\%$ of training questions as the test set, and report performance when training on subsets of the remaining data ranging from $1\%$ to $90\%$ of the full set  We also report the performance on the dev set when trained on the full training set ( $1^\ast $ in Table 2 )  We use the same hyperparameter settings as in prior work  We compare and study four different settings: 1) the Supervised Learning (SL) setting, which is only trained on the supervised data, 2) the best performing GDAN model from BIBREF0 , 3) pretraining on a Language Modeling (LM) objective and fine-tuning on the supervised data, and 4) pretraining on the Cloze dataset and fine-tuning on the supervised data  The LM and Cloze methods use exactly the same data for pretraining, but differ in the loss functions used  We report F1 and EM scores on our test set using the official evaluation scripts provided by the authors of the dataset 
TriviaQA BIBREF4 comprises of over 95K web question-answer-evidence triples  Like SQuAD, the answers are spans of text  Similar to the setting in SQuAD, we create multiple smaller subsets of the entire set  For our semi-supervised QA system, we use the BiDAF+SA model BIBREF1 – the highest performing publicly available system for TrivaQA  Here again, we compare the supervised learning (SL) settings against the pretraining on Cloze set and fine tuning on the supervised set  We report F1 and EM scores on the dev set 
We also test on the BioASQ 5b dataset, which consists of question-answer pairs from PubMed abstracts  We use the publicly available system from BIBREF14 , and follow the exact same setup as theirs, focusing only on factoid and list questions  For this setting, there are only 899 questions for training  Since this is already a low-resource problem we only report results using 5-fold cross-validation on all the available data  We report Mean Reciprocal Rank (MRR) on the factoid questions, and F1 score for the list questions 
Main Results
Table 2 shows a comparison of the discussed settings on both SQuAD and TriviaQA  Without any fine-tuning (column 0) the performance is low, probably because the model never saw a real question, but we see significant gains with Cloze pretraining even with very little labeled data  The BiDAF+SA model, exceeds an F1 score of $50\%$ with only $1\%$ of the training data (454 questions for SQuAD, and 746 questions for TriviaQA), and approaches $90\%$ of the best performance with only $10\%$ labeled data  The gains over the SL setting, however, diminish as the size of the labeled set increases and are small when the full dataset is available 
Cloze pretraining outperforms the GDAN baseline from BIBREF0 using the same SQuAD dataset splits  Additionally, we show improvements in the $90\%$ data case unlike GDAN  Our approach is also applicable in the extremely low-resource setting of $1\%$ data, which we suspect GDAN might have trouble with since it uses the labeled data to do reinforcement learning  Furthermore, we are able to use the same cloze dataset to improve performance on both SQuAD and TriviaQA datasets  When we use the same unlabeled data to pre-train with a language modeling objective, the performance is worse, showing the bias we introduce by constructing clozes is important 
On the BioASQ dataset (Table 3 ) we again see a significant improvement when pretraining with the cloze questions over the supervised baseline  The improvement is smaller than what we observe with SQuAD and TriviaQA datasets – we believe this is because questions are generally more difficult in BioASQ  BIBREF14 showed that pretraining on SQuAD dataset improves the downstream performance on BioASQ  Here, we show a much larger improvement by pretraining on cloze questions constructed in an unsupervised manner from the same domain 
Analysis
Regression Analysis: To understand which types of questions benefit from pre-training, we pre-specified certain features (see Figure 1 right) for each of the dev set questions in SQuAD, and then performed linear regression to predict the F1 score for that question from these features  We predict the F1 scores from the cloze pretrained model ( $y^{\text{cloze}}$ ), the supervised model ( $y^{\text{sl}}$ ), and the difference of the two ( $y^{\text{cloze}}-y^{\text{sl}}$ ), when using $10\%$ of labeled data  The coefficients of the fitted model are shown in Figure 1 (left) along with their std errors  Positive coefficients indicate that a high value of that feature is predictive of a high F1 score, and a negative coefficient indicates that a small value of that feature is predictive of a high F1 score (or a high difference of F1 scores from the two models in the case of $y^{\text{cloze}}-y^{\text{sl}}$ ) 
The two strongest effects we observe are that a high lexical overlap between the question and the sentence containing the answer is indicative of high boost with pretraining, and that a high lexical overlap between the question and the whole passage is indicative of the opposite  This is hardly surprising, since our cloze construction process is biased towards questions which have a similar phrasing to the answer sentences in context  Hence, test questions with a similar property are answered correctly after pretraining, whereas those with a high overlap with the whole passage tend to have lower performance  The pretraining also favors questions with short answers because the cloze construction process produces short answer spans  Also passages and questions which consist of tokens infrequent in the SQuAD training corpus receive a large boost after pretraining, since the unlabeled data covers a larger domain 
Performance on question types: Figure 2 shows the average gain in F1 score for different types of questions, when we pretrain on the clozes compared to the supervised case  This analysis is done on the $10\%$ split of the SQuAD training set  We consider two classifications of each question – one determined on the first word (usually a wh-word) of the question (Figure 2 (bottom)) and one based on the output of a separate question type classifier adapted from BIBREF21   We use the coarse grain labels namely Abbreviation (ABBR), Entity (ENTY), Description (DESC), Human (HUM), Location (LOC), Numeric (NUM) trained on a Logistic Regression classification system   While there is an improvement across the board, we find that abbreviation questions in particular receive a large boost  Also, "why" questions show the least improvement, which is in line with our expectation, since these usually require reasoning or world knowledge which cloze questions rarely require 
Conclusion
In this paper, we show that pre-training QA models with automatically constructed cloze questions improves the performance of the models significantly, especially when there are few labeled examples  The performance of the model trained only on the cloze questions is poor, validating the need for fine-tuning  Through regression analysis, we find that pretraining helps with questions which ask for factual information located in a specific part of the context  For future work, we plan to explore the active learning setup for this task – specifically, which passages and / or types of questions can we select to annotate, such that there is a maximum performance gain from fine-tuning  We also want to explore how to adapt cloze style pre-training to NLP tasks other than QA 
Acknowledgments
Bhuwan Dhingra is supported by NSF under grants CCF-1414030 and IIS-1250956 and by grants from Google  Danish Pruthi and Dheeraj Rajagopal are supported by the DARPA Big Mechanism program under ARO contract W911NF-14-1-0436 
Context 3: Introduction
Machine translation has made remarkable progress, and studies claiming it to reach a human parity are starting to appear BIBREF0  However, when evaluating translations of the whole documents rather than isolated sentences, human raters show a stronger preference for human over machine translation BIBREF1  These findings emphasize the need to shift towards context-aware machine translation both from modeling and evaluation perspective 
Most previous work on context-aware NMT assumed that either all the bilingual data is available at the document level BIBREF2, BIBREF3, BIBREF4, BIBREF5, BIBREF6, BIBREF7, BIBREF8, BIBREF9, BIBREF10 or at least its fraction BIBREF11  But in practical scenarios, document-level parallel data is often scarce, which is one of the challenges when building a context-aware system 
We introduce an approach to context-aware machine translation using only monolingual document-level data  In our setting, a separate monolingual sequence-to-sequence model (DocRepair) is used to correct sentence-level translations of adjacent sentences  The key idea is to use monolingual data to imitate typical inconsistencies between context-agnostic translations of isolated sentences  The DocRepair model is trained to map inconsistent groups of sentences into consistent ones  The consistent groups come from the original training data; the inconsistent groups are obtained by sampling round-trip translations for each isolated sentence 
To validate the performance of our model, we use three kinds of evaluation: the BLEU score, contrastive evaluation of translation of several discourse phenomena BIBREF11, and human evaluation  We show strong improvements for all metrics 
We analyze which discourse phenomena are hard to capture using monolingual data only  Using contrastive test sets for targeted evaluation of several contextual phenomena, we compare the performance of the models trained on round-trip translations and genuine document-level parallel data  Among the four phenomena in the test sets we use (deixis, lexical cohesion, VP ellipsis and ellipsis which affects NP inflection) we find VP ellipsis to be the hardest phenomenon to be captured using round-trip translations 
Our key contributions are as follows:
we introduce the first approach to context-aware machine translation using only monolingual document-level data;
our approach shows substantial improvements in translation quality as measured by BLEU, targeted contrastive evaluation of several discourse phenomena and human evaluation;
we show which discourse phenomena are hard to capture using monolingual data only 
Our Approach: Document-level Repair
We propose a monolingual DocRepair model to correct inconsistencies between sentence-level translations of a context-agnostic MT system  It does not use any states of a trained MT model whose outputs it corrects and therefore can in principle be trained to correct translations from any black-box MT system 
The DocRepair model requires only monolingual document-level data in the target language  It is a monolingual sequence-to-sequence model that maps inconsistent groups of sentences into consistent ones  Consistent groups come from monolingual document-level data  To obtain inconsistent groups, each sentence in a group is replaced with its round-trip translation produced in isolation from context  More formally, forming a training minibatch for the DocRepair model involves the following steps (see also Figure FIGREF9):
sample several groups of sentences from the monolingual data;
for each sentence in a group, (i) translate it using a target-to-source MT model, (ii) sample a translation of this back-translated sentence in the source language using a source-to-target MT model;
using these round-trip translations of isolated sentences, form an inconsistent version of the initial groups;
use inconsistent groups as input for the DocRepair model, consistent ones as output 
At test time, the process of getting document-level translations is two-step (Figure FIGREF10):
produce translations of isolated sentences using a context-agnostic MT model;
apply the DocRepair model to a sequence of context-agnostic translations to correct inconsistencies between translations 
In the scope of the current work, the DocRepair model is the standard sequence-to-sequence Transformer  Sentences in a group are concatenated using a reserved token-separator between sentences  The Transformer is trained to correct these long inconsistent pseudo-sentences into consistent ones  The token-separator is then removed from corrected translations 
Evaluation of Contextual Phenomena
We use contrastive test sets for evaluation of discourse phenomena for English-Russian by BIBREF11  These test sets allow for testing different kinds of phenomena which, as we show, can be captured from monolingual data with varying success  In this section, we provide test sets statistics and briefly describe the tested phenomena  For more details, the reader is referred to BIBREF11 
Evaluation of Contextual Phenomena ::: Test sets
There are four test sets in the suite  Each test set contains contrastive examples  It is specifically designed to test the ability of a system to adapt to contextual information and handle the phenomenon under consideration  Each test instance consists of a true example (a sequence of sentences and their reference translation from the data) and several contrastive translations which differ from the true one only in one specific aspect  All contrastive translations are correct and plausible translations at the sentence level, and only context reveals the inconsistencies between them  The system is asked to score each candidate translation, and we compute the system accuracy as the proportion of times the true translation is preferred to the contrastive ones  Test set statistics are shown in Table TABREF15  The suites for deixis and lexical cohesion are split into development and test sets, with 500 examples from each used for validation purposes and the rest for testing  Convergence of both consistency scores on these development sets and BLEU score on a general development set are used as early stopping criteria in models training  For ellipsis, there is no dedicated development set, so we evaluate on all the ellipsis data and do not use it for development 
Evaluation of Contextual Phenomena ::: Phenomena overview
Deixis Deictic words or phrases, are referential expressions whose denotation depends on context  This includes personal deixis (“I”, “you”), place deixis (“here”, “there”), and discourse deixis, where parts of the discourse are referenced (“that's a good question”)  The test set examples are all related to person deixis, specifically the T-V distinction between informal and formal you (Latin “tu” and “vos”) in the Russian translations, and test for consistency in this respect 
Ellipsis Ellipsis is the omission from a clause of one or more words that are nevertheless understood in the context of the remaining elements  In machine translation, elliptical constructions in the source language pose a problem in two situations  First, if the target language does not allow the same types of ellipsis, requiring the elided material to be predicted from context  Second, if the elided material affects the syntax of the sentence  For example, in Russian the grammatical function of a noun phrase, and thus its inflection, may depend on the elided verb, or, conversely, the verb inflection may depend on the elided subject 
There are two different test sets for ellipsis  One contains examples where a morphological form of a noun group in the last sentence can not be understood without context beyond the sentence level (“ellipsis (infl )” in Table TABREF15)  Another includes cases of verb phrase ellipsis in English, which does not exist in Russian, thus requires predicting the verb when translating into Russian (“ellipsis (VP)” in Table TABREF15) 
Lexical cohesion The test set focuses on reiteration of named entities  Where several translations of a named entity are possible, a model has to prefer consistent translations over inconsistent ones 
Experimental Setup ::: Data preprocessing
We use the publicly available OpenSubtitles2018 corpus BIBREF12 for English and Russian  For a fair comparison with previous work, we train the baseline MT system on the data released by BIBREF11  Namely, our MT system is trained on 6m instances  These are sentence pairs with a relative time overlap of subtitle frames between source and target language subtitles of at least $0 9$ 
We gathered 30m groups of 4 consecutive sentences as our monolingual data  We used only documents not containing groups of sentences from general development and test sets as well as from contrastive test sets  The main results we report are for the model trained on all 30m fragments 
We use the tokenization provided by the corpus and use multi-bleu perl on lowercased data to compute BLEU score  We use beam search with a beam of 4 
Sentences were encoded using byte-pair encoding BIBREF13, with source and target vocabularies of about 32000 tokens  Translation pairs were batched together by approximate sequence length  Each training batch contained a set of translation pairs containing approximately 15000 source tokens  It has been shown that Transformer's performance depends heavily on batch size BIBREF14, and we chose a large batch size to ensure the best performance  In training context-aware models, for early stopping we use both convergence in BLEU score on the general development set and scores on the consistency development sets  After training, we average the 5 latest checkpoints 
Experimental Setup ::: Models
The baseline model, the model used for back-translation, and the DocRepair model are all Transformer base models BIBREF15  More precisely, the number of layers is $N=6$ with $h = 8$ parallel attention layers, or heads  The dimensionality of input and output is $d_{model} = 512$, and the inner-layer of a feed-forward networks has dimensionality $d_{ff}=2048$  We use regularization as described in BIBREF15 
As a second baseline, we use the two-pass CADec model BIBREF11  The first pass produces sentence-level translations  The second pass takes both the first-pass translation and representations of the context sentences as input and returns contextualized translations  CADec requires document-level parallel training data, while DocRepair only needs monolingual training data 
Experimental Setup ::: Generating round-trip translations
On the selected 6m instances we train sentence-level translation models in both directions  To create training data for DocRepair, we proceed as follows  The Russian monolingual data is first translated into English, using the Russian$\rightarrow $English model and beam search with beam size of 4  Then, we use the English$\rightarrow $Russian model to sample translations with temperature of $0{ }5$  For each sentence, we precompute 20 sampled translations and randomly choose one of them when forming a training minibatch for DocRepair  Also, in training, we replace each token in the input with a random one with the probability of $10\%$ 
Experimental Setup ::: Optimizer
As in BIBREF15, we use the Adam optimizer BIBREF16, the parameters are $\beta _1 = 0{ }9$, $\beta _2 = 0{ }98$ and $\varepsilon = 10^{-9}$  We vary the learning rate over the course of training using the formula:
where $warmup\_steps = 16000$ and $scale=4$ 
Results ::: General results
The BLEU scores are provided in Table TABREF24 (we evaluate translations of 4-sentence fragments)  To see which part of the improvement is due to fixing agreement between sentences rather than simply sentence-level post-editing, we train the same repair model at the sentence level  Each sentence in a group is now corrected separately, then they are put back together in a group  One can see that most of the improvement comes from accounting for extra-sentential dependencies  DocRepair outperforms the baseline and CADec by 0 7 BLEU, and its sentence-level repair version by 0 5 BLEU 
Results ::: Consistency results
Scores on the phenomena test sets are provided in Table TABREF26  For deixis, lexical cohesion and ellipsis (infl ) we see substantial improvements over both the baseline and CADec  The largest improvement over CADec (22 5 percentage points) is for lexical cohesion  However, there is a drop of almost 5 percentage points for VP ellipsis  We hypothesize that this is because it is hard to learn to correct inconsistencies in translations caused by VP ellipsis relying on monolingual data alone  Figure FIGREF27(a) shows an example of inconsistency caused by VP ellipsis in English  There is no VP ellipsis in Russian, and when translating auxiliary “did” the model has to guess the main verb  Figure FIGREF27(b) shows steps of generating round-trip translations for the target side of the previous example  When translating from Russian, main verbs are unlikely to be translated as the auxiliary “do” in English, and hence the VP ellipsis is rarely present on the English side  This implies the model trained using the round-trip translations will not be exposed to many VP ellipsis examples in training  We discuss this further in Section SECREF34 
Table TABREF28 provides scores for deixis and lexical cohesion separately for different distances between sentences requiring consistency  It can be seen, that the performance of DocRepair degrades less than that of CADec when the distance between sentences requiring consistency gets larger 
Results ::: Human evaluation
We conduct a human evaluation on random 700 examples from our general test set  We picked only examples where a DocRepair translation is not a full copy of the baseline one 
The annotators were provided an original group of sentences in English and two translations: baseline context-agnostic one and the one corrected by the DocRepair model  Translations were presented in random order with no indication which model they came from  The task is to pick one of the three options: (1) the first translation is better, (2) the second translation is better, (3) the translations are of equal quality  The annotators were asked to avoid the third answer if they are able to give preference to one of the translations  No other guidelines were given 
The results are provided in Table TABREF30  In about $52\%$ of the cases annotators marked translations as having equal quality  Among the cases where one of the translations was marked better than the other, the DocRepair translation was marked better in $73\%$ of the cases  This shows a strong preference of the annotators for corrected translations over the baseline ones 
Varying Training Data
In this section, we discuss the influence of the training data chosen for document-level models  In all experiments, we used the DocRepair model 
Varying Training Data ::: The amount of training data
Table TABREF33 provides BLEU and consistency scores for the DocRepair model trained on different amount of data  We see that even when using a dataset of moderate size (e g , 5m fragments) we can achieve performance comparable to the model trained on a large amount of data (30m fragments)  Moreover, we notice that deixis scores are less sensitive to the amount of training data than lexical cohesion and ellipsis scores  The reason might be that, as we observed in our previous work BIBREF11, inconsistencies in translations due to the presence of deictic words and phrases are more frequent in this dataset than other types of inconsistencies  Also, as we show in Section SECREF7, this is the phenomenon the model learns faster in training 
Varying Training Data ::: One-way vs round-trip translations
In this section, we discuss the limitations of using only monolingual data to model inconsistencies between sentence-level translations  In Section SECREF25 we observed a drop in performance on VP ellipsis for DocRepair compared to CADec, which was trained on parallel data  We hypothesized that this is due to the differences between one-way and round-trip translations, and now we test this hypothesis  To do so, we fix the dataset and vary the way in which the input for DocRepair is generated: round-trip or one-way translations  The latter assumes that document-level data is parallel, and translations are sampled from the source side of the sentences in a group rather than from their back-translations  For parallel data, we take 1 5m parallel instances which were used for CADec training and add 1m instances from our monolingual data  For segments in the parallel part, we either sample translations from the source side or use round-trip translations  The results are provided in Table TABREF35 
The model trained on one-way translations is slightly better than the one trained on round-trip translations  As expected, VP ellipsis is the hardest phenomena to be captured using round-trip translations, and the DocRepair model trained on one-way translated data gains 6% accuracy on this test set  This shows that the DocRepair model benefits from having access to non-synthetic English data  This results in exposing DocRepair at training time to Russian translations which suffer from the same inconsistencies as the ones it will have to correct at test time 
Varying Training Data ::: Filtering: monolingual (no filtering) or parallel
Note that the scores of the DocRepair model trained on 2 5m instances randomly chosen from monolingual data (Table TABREF33) are different from the ones for the model trained on 2 5m instances combined from parallel and monolingual data (Table TABREF35)  For convenience, we show these two in Table TABREF36 
The domain, the dataset these two data samples were gathered from, and the way we generated training data for DocRepair (round-trip translations) are all the same  The only difference lies in how the data was filtered  For parallel data, as in the previous work BIBREF6, we picked only sentence pairs with large relative time overlap of subtitle frames between source-language and target-language subtitles  This is necessary to ensure the quality of translation data: one needs groups of consecutive sentences in the target language where every sentence has a reliable translation 
Table TABREF36 shows that the quality of the model trained on data which came from the parallel part is worse than the one trained on monolingual data  This indicates that requiring each sentence in a group to have a reliable translation changes the distribution of the data, which might be not beneficial for translation quality and provides extra motivation for using monolingual data 
Learning Dynamics
Let us now look into how the process of DocRepair training progresses  Figure FIGREF38 shows how the BLEU scores with the reference translation and with the baseline context-agnostic translation (i e  the input for the DocRepair model) are changing during training  First, the model quickly learns to copy baseline translations: the BLEU score with the baseline is very high  Then it gradually learns to change them, which leads to an improvement in BLEU with the reference translation and a drop in BLEU with the baseline  Importantly, the model is reluctant to make changes: the BLEU score between translations of the converged model and the baseline is 82 5  We count the number of changed sentences in every 4-sentence fragment in the test set and plot the histogram in Figure FIGREF38  In over than 20$\%$ of the cases the model has not changed base translations at all  In almost $40\%$, it modified only one sentence and left the remaining 3 sentences unchanged  The model changed more than half sentences in a group in only $14\%$ of the cases  Several examples of the DocRepair translations are shown in Figure FIGREF43 
Figure FIGREF42 shows how consistency scores are changing in training  For deixis, the model achieves the final quality quite quickly; for the rest, it needs a large number of training steps to converge 
Related Work
Our work is most closely related to two lines of research: automatic post-editing (APE) and document-level machine translation 
Related Work ::: Automatic post-editing
Our model can be regarded as an automatic post-editing system – a system designed to fix systematic MT errors that is decoupled from the main MT system  Automatic post-editing has a long history, including rule-based BIBREF17, statistical BIBREF18 and neural approaches BIBREF19, BIBREF20, BIBREF21 
In terms of architectures, modern approaches use neural sequence-to-sequence models, either multi-source architectures that consider both the original source and the baseline translation BIBREF19, BIBREF20, or monolingual repair systems, as in BIBREF21, which is concurrent work to ours  True post-editing datasets are typically small and expensive to create BIBREF22, hence synthetic training data has been created that uses original monolingual data as output for the sequence-to-sequence model, paired with an automatic back-translation BIBREF23 and/or round-trip translation as its input(s) BIBREF19, BIBREF21 
While previous work on automatic post-editing operated on the sentence level, the main novelty of this work is that our DocRepair model operates on groups of sentences and is thus able to fix consistency errors caused by the context-agnostic baseline MT system  We consider this strategy of sentence-level baseline translation and context-aware monolingual repair attractive when parallel document-level data is scarce 
For training, the DocRepair model only requires monolingual document-level data  While we create synthetic training data via round-trip translation similarly to earlier work BIBREF19, BIBREF21, note that we purposefully use sentence-level MT systems for this to create the types of consistency errors that we aim to fix with the context-aware DocRepair model  Not all types of consistency errors that we want to fix emerge from a round-trip translation, so access to parallel document-level data can be useful (Section SECREF34) 
Related Work ::: Document-level NMT
Neural models of MT that go beyond the sentence-level are an active research area BIBREF2, BIBREF3, BIBREF4, BIBREF5, BIBREF6, BIBREF7, BIBREF8, BIBREF10, BIBREF9, BIBREF11  Typically, the main MT system is modified to take additional context as its input  One limitation of these approaches is that they assume that parallel document-level training data is available 
Closest to our work are two-pass models for document-level NMT BIBREF24, BIBREF11, where a second, context-aware model takes the translation and hidden representations of the sentence-level first-pass model as its input  The second-pass model can in principle be trained on a subset of the parallel training data BIBREF11, somewhat relaxing the assumption that all training data is at the document level 
Our work is different from this previous work in two main respects  Firstly, we show that consistency can be improved with only monolingual document-level training data  Secondly, the DocRepair model is decoupled from the first-pass MT system, which improves its portability 
Conclusions
We introduce the first approach to context-aware machine translation using only monolingual document-level data  We propose a monolingual DocRepair model to correct inconsistencies between sentence-level translations  The model performs automatic post-editing on a sequence of sentence-level translations, refining translations of sentences in context of each other  Our approach results in substantial improvements in translation quality as measured by BLEU, targeted contrastive evaluation of several discourse phenomena and human evaluation  Moreover, we perform error analysis and detect which discourse phenomena are hard to capture using only monolingual document-level data  While in the current work we used text fragments of 4 sentences, in future work we would like to consider longer contexts 
Acknowledgments
We would like to thank the anonymous reviewers for their comments  The authors also thank David Talbot and Yandex Machine Translation team for helpful discussions and inspiration  Ivan Titov acknowledges support of the European Research Council (ERC StG BroadSem 678254) and the Dutch National Science Foundation (NWO VIDI 639 022 518)  Rico Sennrich acknowledges support from the Swiss National Science Foundation (105212_169888), the European Union’s Horizon 2020 research and innovation programme (grant agreement no 825460), and the Royal Society (NAF\R1\180122) 

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Please answer the questions based on the text provided below. how many humans evaluated the results?
Context 4:  For example, there are over 1,000 speakers and over 10 accents in the French and German development / test sets  This enables good coverage of speech variations in both model training and evaluation 
Data Analysis ::: Speaker Diversity
As we can see from Table TABREF2, CoVoST is diversified with a rich set of speakers and accents  We further inspect the speaker demographics in terms of sample distributions with respect to speaker counts, accent counts and age groups, which is shown in Figure FIGREF6, FIGREF7 and FIGREF8  We observe that for 8 of the 11 languages, at least 60% of the sentences (transcripts) are covered by multiple speakers  Over 80% of the French sentences have at least 3 speakers  And for German sentences, even over 90% of them have at least 5 speakers  Similarly, we see that a large portion of sentences are spoken in multiple accents for French, German, Dutch and Spanish  Speakers of each language also spread widely across different age groups (below 20, 20s, 30s, 40s, 50s, 60s and 70s) 
Baseline Results
We provide baselines using the official train-development-test split on the following tasks: automatic speech recognition (ASR), machine translation (MT) and speech translation (ST) 
Baseline Results ::: Experimental Settings ::: Data Preprocessing
We convert raw MP3 audio files from CoVo and TT into mono-channel waveforms, and downsample them to 16,000 Hz  For transcripts and translations, we normalize the punctuation, we tokenize the text with sacreMoses and lowercase it  For transcripts, we further remove all punctuation markers except for apostrophes  We use character vocabularies on all the tasks, with 100% coverage of all the characters  Preliminary experimentation showed that character vocabularies provided more stable training than BPE  For MT, the vocabulary is created jointly on both transcripts and translations  We extract 80-channel log-mel filterbank features, computed with a 25ms window size and 10ms window shift using torchaudio  The features are normalized to 0 mean and 1 0 standard deviation  We remove samples having more than 3,000 frames or more than 256 characters for GPU memory efficiency (less than 25 samples are removed for all languages) 
Baseline Results ::: Experimental Settings ::: Model Training
Our ASR and ST models follow the architecture in berard2018end, but have 3 decoder layers like that in pino2019harnessing  For MT, we use a Transformer base architecture BIBREF15, but with 3 encoder layers, 3 decoder layers and 0 3 dropout  We use a batch size of 10,000 frames for ASR and ST, and a batch size of 4,000 tokens for MT  We train all models using Fairseq BIBREF20 for up to 200,000 updates  We use SpecAugment BIBREF21 for ASR and ST to alleviate overfitting 
Baseline Results ::: Experimental Settings ::: Inference and Evaluation
We use a beam size of 5 for all models  We use the best checkpoint by validation loss for MT, and average the last 5 checkpoints for ASR and ST  For MT and ST, we report case-insensitive tokenized BLEU BIBREF22 using sacreBLEU BIBREF23  For ASR, we report word error rate (WER) and character error rate (CER) using VizSeq 
Baseline Results ::: Automatic Speech Recognition (ASR)
For simplicity, we use the same model architecture for ASR and ST, although we do not leverage ASR models to pretrain ST model encoders later  Table TABREF18 shows the word error rate (WER) and character error rate (CER) for ASR models  We see that French and German perform the best given they are the two highest resource languages in CoVoST  The other languages are relatively low resource (especially Turkish and Swedish) and the ASR models are having difficulties to learn from this data 
Baseline Results ::: Machine Translation (MT)
MT models take transcripts (without punctuation) as inputs and outputs translations (with punctuation)  For simplicity, we do not change the text preprocessing methods for MT to correct this mismatch  Moreover, this mismatch also exists in cascading ST systems, where MT model inputs are the outputs of an ASR model  Table TABREF20 shows the BLEU scores of MT models  We notice that the results are consistent with what we see from ASR models  For example thanks to abundant training data, French has a decent BLEU score of 29 8/25 4  German doesn't perform well, because of less richness of content (transcripts)  The other languages are low resource in CoVoST and it is difficult to train decent models without additional data or pre-training techniques 
Baseline Results ::: Speech Translation (ST)
CoVoST is a many-to-one multilingual ST corpus  While end-to-end one-to-many and many-to-many multilingual ST models have been explored very recently BIBREF8, BIBREF9, many-to-one multilingual models, to our knowledge, have not  We hence use CoVoST to examine this setting  Table TABREF22 and TABREF23 show the BLEU scores for both bilingual and multilingual end-to-end ST models trained on CoVoST  We observe that combining speeches from multiple languages is consistently bringing gains to low-resource languages (all besides French and German)  This includes combinations of distant languages, such as Ru+Fr, Tr+Fr and Zh+Fr  Moreover, some combinations do bring gains to high-resource language (French) as well: Es+Fr, Tr+Fr and Mn+Fr  We simply provide the most basic many-to-one multilingual baselines here, and leave the full exploration of the best configurations to future work  Finally, we note that for some language pairs, absolute BLEU numbers are relatively low as we restrict model training to the supervised data  We encourage the community to improve upon those baselines, for example by leveraging semi-supervised training 
Baseline Results ::: Multi-Speaker Evaluation
In CoVoST, large portion of transcripts are covered by multiple speakers with different genders, accents and age groups  Besides the standard corpus-level BLEU scores, we also want to evaluate model output variance on the same content (transcript) but different speakers  We hence propose to group samples (and their sentence BLEU scores) by transcript, and then calculate average per-group mean and average coefficient of variation defined as follows:
and
where $G$ is the set of sentence BLEU scores grouped by transcript and $G^{\prime } = \lbrace g | g\in G, |g|>1, \textrm {Mean}(g) > 0 \rbrace $ 
$\textrm {BLEU}_{MS}$ provides a normalized quality score as oppose to corpus-level BLEU or unnormalized average of sentence BLEU  And $\textrm {CoefVar}_{MS}$ is a standardized measure of model stability against different speakers (the lower the better)  Table TABREF24 shows the $\textrm {BLEU}_{MS}$ and $\textrm {CoefVar}_{MS}$ of our ST models on CoVoST test set  We see that German and Persian have the worst $\textrm {CoefVar}_{MS}$ (least stable) given their rich speaker diversity in the test set and relatively small train set (see also Figure FIGREF6 and Table TABREF2)  Dutch also has poor $\textrm {CoefVar}_{MS}$ because of the lack of training data  Multilingual models are consistantly more stable on low-resource languages  Ru+Fr, Tr+Fr, Fa+Fr and Zh+Fr even have better $\textrm {CoefVar}_{MS}$ than all individual languages 
Conclusion
We introduce a multilingual speech-to-text translation corpus, CoVoST, for 11 languages into English, diversified with over 11,000 speakers and over 60 accents  We also provide baseline results, including, to our knowledge, the first end-to-end many-to-one multilingual model for spoken language translation  CoVoST is free to use with a CC0 license, and the additional Tatoeba evaluation samples are also CC-licensed 

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