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<Paper uid="P97-1014">
  <Title>Centering in-the-Large: Computing Referential Discourse Segments</Title>
  <Section position="2" start_page="0" end_page="0" type="intro">
    <SectionTitle>
1 Introduction
</SectionTitle>
    <Paragraph position="0"> The centering model (Grosz et al., 1995) has evolved as a major methodology for computational discourse analysis. It provides simple, yet powerful data structures, constraints and rules for the local coherence of discourse. As far as anaphora resolution is concerned, e.g., the model requires to consider those discourse entities as potential antecedents for anaphoric expressions in the current utterance Ui, which are available in the forward-looking centers of the immediately preceding utterance Ui- 1. No constraints or rules are formulated, however, that account for anaphoric relationships which spread out over non-adjacent utterances. Hence, it is unclear how discourse elements which appear in utterances preceding utterance Ui-1 are taken into consideration as potential antecedents for anaphoric expressions in Ui.</Paragraph>
    <Paragraph position="1"> The extension of the search space for antecedents is by no means a trivial enterprise. A simple linear backward search of all preceding centering structures, e.g., may not only turn out to establish illegal references but also contradicts the cognitive principles underlying the limited attention constraint (Walker, 1996b). The solution we propose starts from the observation that additional constraints on valid antecedents are placed by the global discourse structure previous utterances are embedded in.</Paragraph>
    <Paragraph position="2"> We want to emphasize from the beginning that our proposal considers only the referential properties underlying the global discourse structure. Accordingly, we define the extension of referential discourse segments (over several utterances) and a hierarchy of referential discourse segments (structuring the entire discourse). 1 The algorithmic procedure we propose for creating and managing such segments receives local centering data as input and generates a sort of superimposed index structure by which the reachability of potential antecedents, in particular those prior to the immediately preceding utterance, is made explicit. The adequacy of this definition is judged by the effects centered discourse segmentation has on the validity of anaphora resolution (cf. Section 5 for a discussion of evaluation results).</Paragraph>
  </Section>
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