Generating Anaphoric Expressions: Pronoun or Definite Description?

Q @ @ @ O O O @ @ O @ O O @ @ 0 @ @ O @ O @ O O O @ @ @ O @ O O @ @ O O @ @ O @ O O O O Generating Anaphoric Expressions: Pronoun or Definite Description? Kathleen F. McCoy Dept. of Computer and Information Sciences University of Delaware 103 Smith Hall Newark, DE 19716, USA mccoyeci s . udel. edu Michael Strube institute for Research in Cognitive Science University of Pennsylvania 3401 Walnut Street. Suite 400A Philadelphia, PA 19104, USA • strubeglinc, cis .upenn. edu Abstract In order to produce coherent text. natural lan- guage generation systems must have the ability to generate pronouns in the appropriate places. In the past, pronoun usage was primarily inves- tigated with respect to the accessibility of ref- erents. We.argue that generating appropriate referring expressions requires looking at fac- tors beyond accessibility. Also important are sentence boundaries, distance from last men- tion, discourse structure and ambiguity. We present an algorithm for generating appropri- ate anaphoric expressions which takes the tent+ poral structure of texts and knowledge about ambiguous contexts into account. We back up our hypotheses with some empirical results in- dicating that our algorithm chooses the right referring expression in 85% of the cases. 1 Introduction Anaphoric expressions are an important component to generating coherent discourses. While there has been some work on generating appmwiate refening expres- sions, little attention has been given to the problem of when a pronoun should be used to ref= to an object. In most instances the assumption has been that a pronoun should be generated when referring to a discourse entity that is highly prominent (accessible). However, a study of naturally occurring texts reveals that factors beyond accessibility must be ~g*n into account in order to ex- plain the patterns of pronoun use found. Other researchers have indicated that fuller descrip- tions tend to bbe found at the beginning of discourse seg- ments (Grosz & Sidaer, 1986; Reichman, 1985), even when the object being referred to is extzemely prominent in the preceding sentence. There may be two reasons for this: (1) it could be that the item is not accessible since the "focus space" associated with the previous sentence is '*popped" at the boundary (and perhaps an older fo- cus space, with respect to which accessibility should be judged, is restored in its place (Passonneau. 1996)), (2) the use of a fuller definite description is actually mark- ing the discourse segment boundary. If (2) is the case, because other methods for marking boundaries are also possible (and the writer need not use multiple markings), this may help explain why the correlation between dis- course segment boundaries and fuller referring expres- sions is not perfecL Supposing that discourse, segments are an important factor in choosing anaphoric expressions, in order to take advantage of them we must have a clear definition-Of what a discourse segment is. For generation, the dis- course segment boundary must …

Content truncated to 500 words