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Annotating Zero Anaphora for Question Answering

We constructed a large annotated dataset of zero pronouns that correspond to adjuncts marked by -de (translated to English as 'in', 'at', 'by' or 'with') in Japanese. Adjunct zero anaphora resolution plays an important role in extracting information such as location and means from a text. To our knowledge, however, there have been no large-scale dataset covering them. In this paper, focusing on the application of zero anaphora resolution to question answering (QA), we proposed two annotation schemes. The first scheme was designed to efficiently collect zero anaphora instances that are useful in QA. Instead of directly annotating zero anaphora, annotators evaluated QA instances whose correctness hinges on zero anaphora resolution. Over 20,000 instances of zero anaphora were collected with this scheme. We trained a multi-column convolutional neural network with the annotated data, achieving an average precision of 0.519 in predicting the correctness of QA instances of the same type. In the second scheme, zero anaphora is annotated in a more direct manner. A model trained with the results of the second annotation scheme performed better than the first scheme in identifying zero anaphora for sentences randomly sampled from a corpus, suggesting a tradeoff between application-specific and general-purpose annotation schemes.