Structural Ambiguity and Conceptual Relations

Lexical co-occurrence statistics are becoming widely used in the syntactic analysis of unconstrained text. However, analyses based solely on lexical relationships suffer from sparseness of data: it is sometimes necessary to use a less informed model in order to reliably estimate statistical parameters. For example, the "lexical association" strategy for resolving ambiguous prepositional phrase attachments [Hindle and Rooth. 1991] takes into account only the attachment site (a verb or its direct object) and the preposition, ignoring the object of the preposition. We investigated an extension of the lexical association strategy to make use of noun class information, thus permitting a disambiguation strategy to take more information into account. Although in preliminary experiments the extended strategy did not yield improved performance over lexical association alone. a qualitative analysis of the results suggests that the problem lies not in the noun class information, but rather in the multiplicity of classes available for each noun in the absence of sense disambiguation. This suggests several possible revisions of our proposal. 1. P r e f e r e n c e S t r a t e g i e s Prepositional phrase attachment is a paradigmatic case of the structural ambiguity problems faced by natural language parsing systems. Most models of grammar will not constrain the analysis of such attachments in examples like (1): the grammar simply specifies that a prepositional phrase such as on computer theft can be attached in several ways, and leaves the problem of selecting the correct choice to some other process. (1) a. Eventually, Mr. Stoll was invited to both the CIA and NSA to brief high-ranking officers on computer theft. b. Eventually, Mr. Stoll was invited to both the ClA and NSA [to brief [high-ranking officers on computer theft]]. c. Eventually, Mr. Stoll was invited to both the CIA and NSA [to brief [high-ranking ollicers] [on computer theft]]. As [Church and Patil, 1982] point out, the number of analyses given combinations of such "all ways ambiguous" constructions grows rapidly even for sentences of quite Marti A. Hearst Computer Science Division 465 Evans Hall University of California, Berkeley Berkeley, CA 94720 USA mar t i @ c s . b e r k e l e y . e d u reasonable length, so this other process has an important role to play. Discussions of sentence processing have focused primarily on structurally-based preference strategies such as right association and minimal attachment [Kimball, 1973; Frazier, 1979; Ford et al., 1982]; [Hobbs and Bear, 1990], while acknowledging the importance of semantics and pragmatics in attachment decisions, propose two syntactically-based attachment rules that are meant to be generalizations of those structural strategies. Others, however, have argued that syntactic considerations alone are insumcient for determining prepositional phrase attachments, suggesting instead that preference relationships among lexical items are the crucial factor. For example: [Wilks et aL, 1985] argue that the right attachment rules posited by [Frazier, 1979] are incorrect for phrases in general, and supply counterexarnples. They further argue that lexical preferences alone as suggested by [Ford et al., 1982] are too simplistic, and suggest instad the use of preference semantics. In the preference semantics framework, attachment relations of phrases are determined by comparing the preferences emanating from all the entities involved in the attachment, until the best mutual fit is found. Their CASSEX system represents the various meanings of the preposition in terms of (a) the preferred semantic class of the noun or verb that proceeds the preposition (e.g., move, be, strike), (b) the case of the preposition (e.g., instrument, time, loc.static), and (c) the preferred semantic class of the head noun of the prepositional phrase (e.g., physob, event). The difficult part of this method is the identification of preference relationships and particularly determining the strengths of the preferences and how they should interact. (See also discussion in [Schubert, 19841.) lDahlgren and McDowell, 1986] also suggests using preferences based on hand-built knowledge about the prepositions and their objects, specifying a simpler set of rules than those of [Wilks et al., 1985].