Ellipsis Resolution as Question Answering: An Evaluation

Proceedings of the 16th Conference of the European Chapter of the Association for Computational Linguistics, pages 810–817 April 19 - 23, 2021. ©2021 Association for Computational Linguistics 810 Ellipsis Resolution as Question Answering: An Evaluation Rahul Aralikatte1, Matthew Lamm2, Daniel Hardt3, and Anders Søgaard1 1University of Copenhagen, 2Stanford University 3Copenhagen Business School Abstract Most, if not all forms of ellipsis (e.g., ‘so does Mary’) are similar to reading compre- hension questions (‘what does Mary do’), in that in order to resolve them, we need to identify an appropriate text span in the pre- ceding discourse. Following this observation, we present an alternative approach for En- glish ellipsis resolution relying on architec- tures developed for question answering (QA). We present both single-task models, and joint models trained on auxiliary QA and corefer- ence resolution datasets, clearly outperform- ing the current state of the art for Sluice Ellip- sis (from 70.00 to 86.01 F1) and Verb Phrase Ellipsis (from 72.89 to 78.66 F1). 1 Introduction Ellipsis resolution is a hard, open problem in NLP, and an important source of error in machine trans- lation, question answering, and dialogue under- standing (Vicedo and Ferr´andez, 2000; Dzikovska et al., 2009; Chung and Gildea, 2010; Macketanz et al., 2018; Petrn Bach Hansen and Sgaard, 2020). There are no large annotated text corpora for this phenomenon, even for English, and we only have annotations for a subset of the known ellipsis con- structions. Since annotation is expensive and cum- bersome, any synergies with existing NLP tasks could be useful and enable us to leverage auxiliary data when learning models for ellipsis resolution. This paper presents a simple yet strong ap- proach to ellipsis resolution based on a straightfor- ward observation, depicted in Figure 1, that ellip- sis resolution can be converted to a QA problem. Ellipsis and questions put in focus referentially de- pendent expressions (Carlson, 2006), or free vari- ables (Partee, 1978), that need to be resolved in or- der to comprehend the discourse. For similar ob- servations about different tasks, see McCann et al. (2018) and Gardner et al. (2019). This straightforward observation leads us to Sluice Ellipsis Context: … But the way things are structured now you have to set aside your ego to make things happen. The whole thing worked out. I don't know how, but it did. Both sides had to work to make it happen … Question: I don't know how, but it did. Answer: The whole thing worked out Verb Phrase Ellipsis Context: … It has to be considered as an additional risk for the investor," said Gary P. Smaby of Smaby Group Inc., Minneapolis. "Cray Computer will be a concept stock," he said. "You either believe Seymour can do it again or you don't … Question: You either believe Seymour can do it again or you don't. Answer: believe Seymour can do it again Figure 1: Examples of Sluice Ellipsis and Verb Phrase Ellipsis, represented as “questions” about their asso- ciated contexts. Wh-phrases and auxiliary verbs are marked …

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