Structural Ambiguity and Lexical Relations

STRUCTURAL AMBIGUITY AND LEXICAL RELATIONS Donald Hindle and Mats Rooth AT&T Bell Laboratories 600 Mountain Avenue Murray Hill, NJ 07974 Abstract We propose that ambiguous prepositional phrase attachment can be resolved on the basis of the relative strength of association of the preposition with noun and verb, estimated on the basis of word distribution in a large corpus. This work suggests that a distributional approach can be effective in resolving parsing problems that apparently call for complex reasoning. Introduction Prepositional phrase attachment is the canonical case of structural ambiguity, as in the time worn example, (1) I saw the man with the telescope The existence of such ambiguity raises problems for understanding and for language models. It looks like it might require extremely complex com- putation to determine what attaches to what. In- deed, one recent proposal suggests that resolving attachment ambiguity requires the construction of a discourse model in which the entities referred to in a text must be reasoned about (Altmann and Steedman 1988). Of course, if attachment am- biguity demands reference to semantics and dis- course models, there is little hope in the near term of building computational models for unrestricted text to resolve the ambiguity. Structure based ambiguity resolution There have been several structure-based proposals about ambiguity resolution in the literature; they are particularly attractive because they are simple and don't demand calculations in the semantic or discourse domains. The two main ones are: • Right Association - a constituent tends to at- tach to another constituent immediately to its right (Kimball 1973). • Minimal Attachment - a constituent tends to attach so as to involve the fewest additional syntactic nodes (Frazier 1978). For the particular case we are concerned with, attachment of a prepositional phrase in a verb + object context as in sentence (1), these two princi- ples - at least in the version of syntax that Frazier assumes - make opposite predictions: Right Asso- ciation predicts noun attachment, while Minimal Attachment predicts verb attachment. Psycholinguistic work on structure-based strate- gies is primarily concerned with modeling the time course of parsing and disambiguation, and propo- nents of this approach explicitly acknowledge that other information enters into determining a final parse. Still, one can ask what information is rel- evant to determining a final parse, and it seems that in this domain structure-based disambigua- tion is not a very good predictor. A recent study of attachment of prepositional phrases in a sam- ple of written responses to a "Wizard of Oz" travel information experiment shows that neither Right Association nor Minimal Attachment account for more than 55% of the cases (Whittemore et al. 1990). And experiments by Taraban and McClel- land (1988) show that the structural models are not in fact good predictors of people's behavior in resolving ambiguity. Resolving ambiguity through lexical associations Whittemore et al. (1990) found lexical preferences to be the key to resolving attachment ambiguity. Similarly, Taraban and McClelland found lexical content was key in explaining people's behavior. Various previous proposals …

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