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<Paper uid="W90-0115">
  <Title>The Basic Block Model of Extended Explanations</Title>
  <Section position="2" start_page="0" end_page="0" type="intro">
    <SectionTitle>
INTRODUCTION
</SectionTitle>
    <Paragraph position="0"> In this paper, we describe the high-level structure of naturally-occurring, extended explanations and contend that the organizational strategies currently employed for structuring short explanations are inadequate for generating this characteristic high-level structure. Our analysis suggests that text structure is not completely recursive as others have claimed (\[Grosz and Sidner, 1986\], \[Reichman, 1978\], \[Polanyi, 1986\], \[Mann and Thompson, 1988\]), but rather that the high-level structure of extended explanations is determined by processes separate from those which organize text at lower levels. Evidence for the non-recursive nature of this structure is provided by repetition, which is frequently used in extended explanations for a wide variety of purposes. This device, whose use has been neglected by current explanation research, exhibits a distinct relationship to the structure of extended explanations, implying that it, too, cannot be generated by recursive strategies.</Paragraph>
    <Paragraph position="1"> Our analysis is based on several sources, the primary of which is the testimony before congressional committees regarding the Three Mile Island (TMI) accident (\[Representatives, 1979\], \[Representatives, 1980\]). These explanations were punctuated with numerous interruptions and characteristically involved the operation of complex physical devices and extended causal chains of events. The majority of these explanations were planned beforehand and required many pages of text - typically five, ten, or more, exclusive of interruptions. Furthermore, the testimony comes from a variety of sources: operators who were on the scene as the accident developed; engineers who gave post-accident evaluations; executives from the company that designed the plant. This variety allowed examination of explanations of the same set of events from many perspectives.</Paragraph>
    <Paragraph position="2"> A secondary source was transcripts of a medical class involved in patient diagnosis. While these transcripts differed greatly form those of TMI - in particular, the explanations provided were relatively short; they were unplanned; and they were concerned with diagnostic strategy as opposed to causal events - they exhibited many of the characteristics found in the TMI explanations. null The next section provides a brief overview of current models for structuring text, followed by a description of the basic block, the unit of discourse on which our model of discourse is based. We then identify three types of repetition whose use supports our contentions of the non-recursive nature of basic block structure. We conclude by describing the characteristics of high-level structure of extended explanations, followed by a brief description of our strategy for computationally generating this structure. A more detailed description of our strategy and its role in a complete generational system is contained in \[Mooney, el al. 1989\].</Paragraph>
  </Section>
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