Recent work on open domain question answering (QA) assumes strong supervision of the supporting evidence and/or assumes a blackbox information retrieval (IR) system to retrieve evidence candidates. We argue that both are suboptimal, since gold evidence is not always available, and QA is fundamentally different from IR. We show for the first time that it is possible to jointly learn the retriever and reader from question-answer string pairs and without any IR system. In this setting, evidence retrieval from all of Wikipedia is treated as a latent variable. Since this is impractical to learn from scratch, we pre-train the retriever with an Inverse Cloze Task. We evaluate on open versions of five QA datasets. On datasets where the questioner already knows the answer, a traditional IR system such as BM25 is sufficient. On datasets where a user is genuinely seeking an answer, we show that learned retrieval is crucial, outperforming BM25 by up to 19 points in exact match.

本文提出了一种新的open domain question answering方法, 利用问题-回答对来同时学习retriever和reader，将Wikipedia中的证据检索视为潜在变量，并通过Inverse Cloze Task进行预训练。我们的方法在五个QA数据集上进行测试，表明在用户真正寻求答案的数据集上，学习检索至关重要，完全匹配方面的得分比传统的IR系统（如BM25）高达19个点。

弱监督开放域问答的潜在检索