A General Computational Treatment of Comparatives for Natural Language Question Answering

A General Computational Treatment of Comparatives for Natural Language Question Answering Bruce W. Ballard AT&T Bell Laborotories 600 Mountain Avenue Murray Hill, N.J. 07974 Abstract We discuss the techniques we have developed and implemented for the cross-categorial treatment of comparatives in TELl, a natural language question- answering system that's transportable among both application domains and types of backend retrieval systems. For purposes of illustration, we shall consider the example sentences "List the cars at least 20 inches more than twice as long as the Century is wide" and "Have any US companies made at least 3 more large cars than Buick?" Issues to be considered include comparative inflections, left recursion and other forms of nesting, extraposition of comparative complements, ellipsis, the wh element "how', and the translation of normalized parse trees into logical form. 1. Introduction We shall describe a general treatment of comparatives that has been implemented in the context of TELI, a question-answering system which is transportable among both domains of discourse and different types of backend retrieval systems.n Comparatives are important because of the dramatic increase in expressive power they allow; they are interesting at least because of the variety of issues (from morphology on up) one must deal with in order to provide for them. 1. The examples in this paper illustrate TEL1 us a front-end to the Kandor knowledge representation system (Patel-Schneider, 1984); we will give examples in terms of a knowledge base of information about 1987 cars. TELI has produced queries for at least four different "backend" systems and has been adapted for over a dozen domains of data. 41 1.1 Goals In seeking to provide TEL1 with general capabilities for comparatives, our primary goals have been to formulate cross-categorial techniques that treat the comparativizations of different syntactic elements (e.g. adjectives, quantifiers, and measure nouns) with the same mechanisms; to allow comparatives to be composed with themselves (e.g. "at least 3 more than 3 times as many') and with other syntactic features (e.g. wh elements); to be faithful to what is known from work in theoretical linguistics; we draw from Bresnan (1973), Cushing (1982), Dik (1980), Jackendoff (1977), Sells (1985), and Winograd (1983); to account for as many of the specific cases of comparatives found in the literatureof implemented NL processors as possible. 1.2 Achievements Letting <X> denote a grammatical category to be comparativized, we begin by providing for comparativized structures C{<X>} of the form C{<X>} -.* (<Qmd>) CC{<X>) <Comp> <Qua> -'* *tmostlatleutlaolexsctlylg~'dmyljastlealy CC{<X>} -=*" (CC{<X>}) (<Measure>) <el> (<X>) <c2> <Measure> --* <Number> (<Ordinai>lperc~tltinNs) I <onus> --* h~lt~ltUrdsl-- <Comp> --0 <NP> <Etcx> <el>/<c2> .-4, -er/flum[less/thu[ss/us where (...) denotes optionality; "/" indicates "agreement" between comparative particles; and <Etcx> accounts for items parallel to those in the matrix clause in which the comparative occurs (e.g. "cars that are longer than the Regal (is (wide))'). In addition, a variety of extrapositions (i.e. rightward and occasional leftward movement) from C{<X>} may (and sometimes must) occur. For example, both "cars larger than the Century" and "larger cars than the …

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