Resolving “This-issue” Anaphora

Proceedings of the 2012 Joint Conference on Empirical Methods in Natural Language Processing and Computational Natural Language Learning, pages 1255–1265, Jeju Island, Korea, 12–14 July 2012. c⃝2012 Association for Computational Linguistics Resolving “This-issue” Anaphora Varada Kolhatkar Department of Computer Science University of Toronto Toronto, ON, M5S 3G4, Canada varada@cs.toronto.edu Graeme Hirst Department of Computer Science University of Toronto Toronto, ON, M5S 3G4, Canada gh@cs.toronto.edu Abstract We annotate and resolve a particular case of abstract anaphora, namely, this- issue anaphora. We propose a candidate ranking model for this-issue anaphora resolution that explores different issue- specific and general abstract-anaphora features. The model is not restricted to nominal or verbal antecedents; rather, it is able to identify antecedents that are arbitrary spans of text. Our re- sults show that (a) the model outperforms the strong adjacent-sentence baseline; (b) general abstract-anaphora features, as distinguished from issue-specific fea- tures, play a crucial role in this-issue anaphora resolution, suggesting that our approach can be generalized for other NPs such as this problem and this debate; and (c) it is possible to reduce the search space in order to improve performance. 1 Introduction Anaphora in which the anaphoric expression refers to an abstract object such as a proposition, a prop- erty, or a fact is known as abstract object anaphora. This is seen in the following examples. (1) [Be careful what you wish... because wishes sometimes come true.]i [That]i is what the Semiconductor Industry Association, which rep- resents U.S. manufacturers, has been learning. (from Asher (1993)) (2) This prospective study suggested [that oral carvedilol is more effective than oral meto- prolol in the prevention of AF after on-pump CABG]i. It is well tolerated when started before and continued after the surgery. However, further prospective studies are needed to clarify [this is- sue]i. (3) In principle, he said, airlines should be allowed [to sell standing-room-only tickets for adults]i — as long as [this decision]i was approved by their marketing departments. These examples highlight a difficulty not found with nominal anaphora. First, the anaphors refer to ab- stract concepts that can be expressed with differ- ent syntactic shapes which are usually not nominals. The anaphor That in (1) refers to the proposition in the previous utterance, whereas the anaphor this is- sue in (2) refers to a clause from the previous text. In (3), the anaphoric expression this decision refers to a verb phrase from the same sentence. Second, the antecedents do not always have precisely defined boundaries. In (2), for example, the whole sentence containing the marked clause could also be thought to be the correct antecedent. Third, the actual refer- ents are not always the precise textual antecedents. The actual referent in (2), the issue to be clarified, is whether oral carvedilol is more effective than oral metoprolol in the prevention of AF after on-pump CABG or not, a variant of the antecedent text. Generally, abstract anaphora, as distinguished from nominal anaphora, is signalled in English by pronouns this, that, and it (M¨uller, 2008). But in abstract …

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