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        <title>A Discrete Hard EM Approach for Weakly Supervised Question Answering</title>
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    <abstract>Many question answering (QA) tasks only provide weak supervision for how the answer should be computed. For example, TriviaQA answers are entities that can be mentioned multiple times in supporting documents, while DROP answers can be computed by deriving many different equations from numbers in the reference text. In this paper, we show it is possible to convert such tasks into discrete latent variable learning problems with a precomputed, task-specific set of possible solutions (e.g. different mentions or equations) that contains one correct option. We then develop a hard EM learning scheme that computes gradients relative to the most likely solution at each update. Despite its simplicity, we show that this approach significantly outperforms previous methods on six QA tasks, including absolute gains of 2–10%, and achieves the state-of-the-art on five of them. Using hard updates instead of maximizing marginal likelihood is key to these results as it encourages the model to find the one correct answer, which we show through detailed qualitative analysis.</abstract>
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    <identifier type="doi">10.18653/v1/D19-1284</identifier>
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%0 Conference Proceedings
%T A Discrete Hard EM Approach for Weakly Supervised Question Answering
%A Min, Sewon
%A Chen, Danqi
%A Hajishirzi, Hannaneh
%A Zettlemoyer, Luke
%S Proceedings of the 2019 Conference on Empirical Methods in Natural Language Processing and the 9th International Joint Conference on Natural Language Processing (EMNLP-IJCNLP)
%D 2019
%8 nov
%I Association for Computational Linguistics
%C Hong Kong, China
%F min-etal-2019-discrete
%X Many question answering (QA) tasks only provide weak supervision for how the answer should be computed. For example, TriviaQA answers are entities that can be mentioned multiple times in supporting documents, while DROP answers can be computed by deriving many different equations from numbers in the reference text. In this paper, we show it is possible to convert such tasks into discrete latent variable learning problems with a precomputed, task-specific set of possible solutions (e.g. different mentions or equations) that contains one correct option. We then develop a hard EM learning scheme that computes gradients relative to the most likely solution at each update. Despite its simplicity, we show that this approach significantly outperforms previous methods on six QA tasks, including absolute gains of 2–10%, and achieves the state-of-the-art on five of them. Using hard updates instead of maximizing marginal likelihood is key to these results as it encourages the model to find the one correct answer, which we show through detailed qualitative analysis.
%R 10.18653/v1/D19-1284
%U https://aclanthology.org/D19-1284
%U https://doi.org/10.18653/v1/D19-1284
%P 2851-2864

Markdown (Informal)

[A Discrete Hard EM Approach for Weakly Supervised Question Answering](https://aclanthology.org/D19-1284) (Min et al., EMNLP 2019)

• A Discrete Hard EM Approach for Weakly Supervised Question Answering (Min et al., EMNLP 2019)

ACL

• Sewon Min, Danqi Chen, Hannaneh Hajishirzi, and Luke Zettlemoyer. 2019. A Discrete Hard EM Approach for Weakly Supervised Question Answering. In Proceedings of the 2019 Conference on Empirical Methods in Natural Language Processing and the 9th International Joint Conference on Natural Language Processing (EMNLP-IJCNLP), pages 2851–2864, Hong Kong, China. Association for Computational Linguistics.