Anaphora Resolution: A Multi-Strategy Approach

Anaphora Resolution: A Multi.Strategy Approach Jaime G. Carbonell Ralf D. Brown Computer Science Department & Center for Machine Translation Carnegie-Mellon University Pittsburgh, PA 15213 15 April 1988 Primary topic: discourse Secondary topics: semantics, computational models, natural language interfaces, pragmatics Abstract Anaphora resolution has proven to be a very difficult problem; it requires the integrated application of syntactic, semantic, and pragmatic knowledge. This paper examines the hypothesis that instead of attempting to construct a monolithic method for resolving anaphora, the combination of multiple strategies, each exploiting a different knowledge source, proves more effective, theoretically and computationally. Cognitive plausibility is established in that human judgements of the optimal anaphoric referent accord with those of the strategy-based method, and human inability to determine a unique referent corresponds to the cases where different strategies offer conflicting candidates for the anaphoric referent. 1, Introduction: The Complexity of Anaphora Resolution Anaphora is a pervasive phenomenon in natural language communication, whether it be complex multi-party human discourse or more constrained bilateral human-computer dialog. Whereas the ubiquity of the phenomenon and tire necessity to cope with it in the construction of friendly natural language interfaces has long been recognized [13, 8, 9, 17, 15], no truly comprehensive computational approaches for anaphora resolution have been proposed. The RUS parser [2], the XCALIBUR system [7], and other operational natural language systems implement very rudimentary methods. And, theoretical work in linguistics is primarily concerned with certain types of syntactic intrasentential coreference, rather than pragmatic intersentential anaphora. AnaPhors typically refer back to other constituents in the same sentence, or to constituents in earlier utterances in fire discourse. Syntactic information plays a central role in establishing appropriate referents for the former case, intrasentential anaphora [17]. But, semantic and pragmatic infonnation is absolutely required in the latter case, intersentential anaphora [15, 9]. This paper addresses the problem of intersentential anaphora resolution, integrating caseframe semantics [10, 12, 5] and more global dialog coherence structures [11, 15, 14]. Empirical studies have shown that intersentential anaphora I is far more frequent and more crucial in designing interactive natural language interfaces 2 [8]. 96 2. The Problem: Semantics and Pragmatics Dominate Finding the appropriate anaphoric referent has been long recognized as a difficult problem, requiring lnuch ~emautic and pragmatic lmowledge. Consider, for instance, the tbllowing two sets of examples: John took the cake from the table and ate it. John took the cake from the table and washed it. Tile robot pushed the box towards the conveyor belt. But, it goojSd and dropped it on its way there. Semantic preference constraints (e.g., [18, 11), if pmp:~rly coded, suffice to resolve the first example. The pt'eferre'l object of ingestation is an edible substance. It is a little more difficult to mechanize a process that excludes things such as cakes from being the object of washing. One cannot simply write a "NOT(edibley restriction on the object case of the verb "to wash". Alter ,all, vegetables and fruits are occasionally washed prior to eating them. Peltlaps a combination of typicality judgements …

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