Taking Time to Structure Discourse: Pronoun Generation Beyond Accessibility

In order to produce coherent text, natural language generation systems must have the ability to generate pronouns in the appropriate places. In the past, pronoun usage was primarily investigated with respect to the accessibility of referents. That is, it was assumed that a pronoun should be generated whenever the referent was sufficiently accessible so as to make its resolution easy. We found that such an explanation does not seem to account well for the patterns of pronoun usage found in naturally occurring texts. We present an algorithm for generating appropriate anaphoric expressions which takes into account the temporal structure of texts (as a discourse structuring device) and knowledge about ambiguous contexts. Other important factors in our algorithm are sentence boundaries and the distance from the last mention of the anaphor. We back up our hypotheses with some empirical results indicating that our algorithm chooses the right referring expression in 85% of the cases.