A Sequencing Model for Situation Entity Classification

Proceedings of the 45th Annual Meeting of the Association of Computational Linguistics, pages 896–903, Prague, Czech Republic, June 2007. c⃝2007 Association for Computational Linguistics A Sequencing Model for Situation Entity Classification Alexis Palmer, Elias Ponvert, Jason Baldridge, and Carlota Smith Department of Linguistics University of Texas at Austin {alexispalmer,ponvert,jbaldrid,carlotasmith}@mail.utexas.edu Abstract Situation entities (SEs) are the events, states, generic statements, and embedded facts and propositions introduced to a discourse by clauses of text. We report on the first data- driven models for labeling clauses according to the type of SE they introduce. SE classifi- cation is important for discourse mode iden- tification and for tracking the temporal pro- gression of a discourse. We show that (a) linguistically-motivated cooccurrence fea- tures and grammatical relation information from deep syntactic analysis improve clas- sification accuracy and (b) using a sequenc- ing model provides improvements over as- signing labels based on the utterance alone. We report on genre effects which support the analysis of discourse modes having charac- teristic distributions and sequences of SEs. 1 Introduction Understanding discourse requires identifying the participants in the discourse, the situations they par- ticipate in, and the various relationships between and among both participants and situations. Coreference resolution, for example, is concerned with under- standing the relationships between references to dis- course participants. This paper addresses the prob- lem of identifying and classifying references to situ- ations expressed in written English texts. Situation entities (SEs) are the events, states, generic statements, and embedded facts and propo- sitions which clauses introduce (Vendler, 1967; Verkuyl, 1972; Dowty, 1979; Smith, 1991; Asher, 1993; Carlson and Pelletier, 1995). Consider the text passage below, which introduces an event-type entity in (1), a report-type entity in (2), and a state- type entity in (3). (1) Sony Corp. has heavily promoted the Video Walkman since the product’s introduction last summer , (2) but Bob Gerson , video editor of This Week in Con- sumer Electronics , says (3) Sony conceives of 8mm as a “family of products , camcorders and VCR decks , ” SE classification is a fundamental component in de- termining the discourse mode of texts (Smith, 2003) and, along with aspectual classification, for tempo- ral interpretation (Moens and Steedman, 1988). It may be useful for discourse relation projection and discourse parsing. Though situation entities are well-studied in lin- guistics, they have received very little computational treatment. This paper presents the first data-driven models for SE classification. Our two main strate- gies are (a) the use of linguistically-motivated fea- tures and (b) the implementation of SE classification as a sequencing task. Our results also provide empir- ical support for the very notion of discourse modes, as we see clear genre effects in SE classification. We begin by discussing SEs in more detail. Sec- tion 3 describes our two annotated data sets and pro- vides examples of each SE type. Section 4 discusses feature sets, and sections 5 and 6 present models, experiments, and results. 896 2 Discourse modes and situation entities In this section, …

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