Shared Preferences

SHARED PREFERENCES James Barnett and Inderjeet Mani MCC 3500 West Balcones Center Dr. Austin, TX 78759 barnett@mcc.com mani@mcc.com !Abstract This paper attempts to develop a theory of heuristics or preferences that can be shared be- tween understanding and generation systems. We first develop a formal analysis of preferences and consider the relation between their uses in gener- ation and understanding. We then present a bi- directional algorithm for applying them and ex- amine typical heuristics for lexical choice, scope and anaphora in:, more detail. 1 Introduction Understanding and generation systems must both deal with ambiguity. In understanding, there are often a number of possible meanings for a string, while there are usually a number of different ways of expressing a given meaning in generation. To control the expl6sion of possibilities, researchers have developed a variety of heuristics or prefer- ences - for example, a preference for low attach- ment of modifiers in understanding or for conci- sion in generation. This paper investigates the possibility of sharing such preferences between understanding and generation as part of a bidirec- tional NL system. In Section 2 we formalize the concept of a preference, and Section 3 presents an algorithm fo ! applying such preferences uni- formly in understanding and generation. In Sec- tion 4 we consider specific heuristics for lexical choice, scope, and anaphora. These heuristics have special properties that permit a more effi- cient implementation than the general algorithm from Section 3. Section 5 discusses some of the short-comings of the theory developed here and suggests directions for future research. 2 Preferences in Under- standing and Generation Natural language understanding is a mapping from utterances to meanings, while generation goes in the opposite direction. Given a set String of input strings (of a given language) and a set Int of interpretations or meanings, we can represent understanding as a relation U C String x Int, and generation as G C IntxString. U and G are relations, rather than functions, since they allow for ambiguity: multiple meanings for an utter- ance and multiple ways of expressing a meaning 1. A minimal requirement for a reversible system is that U and G be inverses of each other. For all s 6 String and i 6 Int: (s, Oeu (i,s)ec (1) Intuitively, preferences are ways of controlling the ambiguity of U and G by ranking some inter- pretations (for U) or strings (for G) more highly than others. Formally, then, we can view prefer- ences as total orders on the objects in question (we will capitalize the term when using it in this technical sense). 2 Thus, for any s 6 String an un- derstanding Preference Pint will order the pairs {(s, 01(s,=) E U}, while a generation Preference * The definitions of U and G allow for strings with no interpretations and meanings with no strings. Since any meaning can presumably be expressed in any language, we may want to further restrict G so that everything is expressible: Yi 6 Int (Bs 6 String …

Content truncated to 500 words