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<Paper uid="W99-0106">
  <Title>Discourse Structure and Co-Reference: An Empirical Study</Title>
  <Section position="11" start_page="52" end_page="52" type="concl">
    <SectionTitle>
4 Conclusion
</SectionTitle>
    <Paragraph position="0"> We an~,lyzed empirically the potentials of discourse and linear models of text to determine co-referential links. Our analysis suggests that by exploiting the hierarchical structure of texts, one can increase the potential of natural language systems to correctly determine co-referential links, which is a requirement for correctly resolving anaphors. If one treats all discourse units in the preceding discourse equally, the increase is statistically significant only when a discourse-based corefererice system looks back at most four discourse units in order to establish co-referenfial links. However, if one assumes that proximity plays an important role in establishing co-referential links and that referential expressions are more likely to be linked to referees that were used recently in discourse, the increase is statistically significant no matter how many units a discourse-based co-reference system looks back in order toestablish co-referenfial links.</Paragraph>
    <Paragraph position="1"> Acknowledgements. We ate grateful to Lynette Hirschman and Nancy Chinchor for making available their corpus of co-reference annotations. We are also grateful to Graeme Hirst for comments and feedback on a previous draft of this paper.</Paragraph>
  </Section>
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