anaphora resolution

anaphora resolution - ANAPHORA RESOLUTION: THE STATE OF THE...

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ANAPHORA RESOLUTION: THE STATE OF THE ART Ruslan Mitkov School of Languages and European Studies University of Wolverhampton Stafford Street Wolverhampton WV1 1SB United Kingdom Email [email protected] http://www.wlv.ac.uk/~le1825/ 1. Introduction Anaphora resolution is a complicated problem in Natural Language Processing and has attracted the attention of many researchers. The approaches developed - traditional (from purely syntactic ones to highly semantic and pragmatic ones), alternative (statistic, uncertainty-reasoning etc.) or knowledge-poor, offer only approximate solutions. The paper is an introduction to anaphora resolution offering a brief survey of the major works in the field. 1.1 Basic notions and terminology The etymology of the term "anaphora" goes back to Ancient Greek with “anaphora” ( αναϕορα29 being a compound word consisting of the separate words ανα - back, upstream, back in an upward direction and ϕορα - the act of carrying and denoted the act of carrying back upstream . For Computational Linguists embarking upon research in the field of anaphor resolution, I strongly recommend as a primer Graham Hirst's book "Anaphora in natural language understanding" (Hirst 1981) which may seem a bit dated in that it does not include developments in the 80's and the 90's, but which provides an excellent survey of the theoretical work on anaphora and of the early com- putational approaches and is still very useful reading. Various definitions of anaphora have been put forward, but I am tempted to paraphrase based on the notion of cohesion: anaphora is cohesion (presupposition) which points back to some previous 1 item. 1 We shall not discuss cataphora which is the case when the "anaphor" precedes the antecedent (e.g. "Because she was going to the post office, Julie was asked to post a small parcel")
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2 The "pointing back" (reference) is called an anaphor and the entity to which it refers is its antecedent . The process of determining the antecedent of an anaphor is called anaphora resolution . Usually, both the antecedent and the anaphor are used as referring expressions and having the same referent 2 in the real world, they are termed coreferential . Example (Huddleston 1984): The Empress hasn't arrived yet but she should be here any minute In this example, the pronoun "she" is the anaphor (for classification of anaphors, see below) and "the Empress" is the antecedent. Please note that the antecedent is not the noun "Empress" but the noun phrase "the Empress". There may be cases when the anaphor and more than one of the preceding (or following) entities (usually noun phrases) have the same referent and are therefore pairwise coreferential, thus forming a coreferential chain. In such a case, we regard each of the preceding entities which are coreferential with the anaphor(s) as a legitimate antecedent. Therefore, in such cases the task of anaphora resolution is considered
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anaphora resolution - ANAPHORA RESOLUTION: THE STATE OF THE...

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