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<Paper uid="P84-1063">
  <Title>A Plan Recognition Model for Clarification Subdialogues</Title>
  <Section position="1" start_page="0" end_page="302" type="abstr">
    <SectionTitle>
Abstract
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    <Paragraph position="0"> One of the promising approaches to analyzing task-oriented dialogues has involved modeling the plans of the speakers in the task domain. In general, these models work well as long as the topic follows the task structure closely, but they have difficulty in accounting for clarification subdialogues and topic change. We have developed a model based on a hierarchy of plans and metaplans that accounts for the clarification subdialogues while maintaining the advantages of the plan-based approach.</Paragraph>
    <Paragraph position="1"> I. Introduction One of the promising approaches to analyzing task-oriented dialogues has involved modeling the plans of the speakers in the task domain. The earliest work in this area involved tracking the topic of a dialogue by tracking the progress of the plan in the task domain \[Grosz, 1977\], as well as explicitly incorporating speech acts into a planning framework \[Cohen and Perrault, 1979; Allen and Perrault, 1980\]. A good example of the current status of these approaches can be found in \[Carberry, 1983\]. In general, these models work well as long as the topic follows the task structure closely, but they have difficulty in accounting for clarification subdialogues and topic change.</Paragraph>
    <Paragraph position="2"> Sidner and Israel \[1981\]suggest a solution to a class of clarification subdialogues that correspond to debugging the plan in the task domain. They allow utterances to talk about the task plan, rather than always being a step in the plan. Using their suggestions, as well as our early work \[Allen et al., 1982: Litman, 1983\], we have developed a model based on a hierarchy of plans and metaplans that This work was supported in part by the National Science Foundation under Grant IST-8210564, the Office of Naval Research under Grant N00014-80-C-1097, and the Defense Advanced Research Projects Agency under Grant N00014-82-K-0193.</Paragraph>
    <Paragraph position="3"> accounts for the debugging subdialogues they discussed, as well as other forms of clarification and topic shi~.</Paragraph>
    <Paragraph position="4"> Reichman \[1981\] has a structural model of discourse that addresses clarification subdialogues and topic switch in unconstrained spontaneous discourse. Unfortunately, there is a large gap between her abstract model and the actual processing of utterances. Although not the focus of this paper, we claim that our new plan recognition model provides the link from the processing of actual input to its abstract discourse structure. Even more important, this allows us to use the linguistic results from such work to guide and be guided by our plan recognition.</Paragraph>
    <Paragraph position="5"> For example, consider the following two dialogue fragments. The first was collected at an information booth in a train station in Toronto \[Horrigan, 1977\], while the second is a scenario developed from protocols in a graphics command and control system that displays network structures \[Sidner and Bates, 1983\].</Paragraph>
    <Paragraph position="6">  1) Passenger: 2) Clerk: 3) Passenger: 4) Clerk: 5) Passenger: 6) User: 7) System: 8) User: 9) System: 10) User: 11) System:  The eight-fifty to Montreal? Eight-fifty to Montreal. Gate seven.</Paragraph>
    <Paragraph position="7"> Where is it? Down this way to the left. Second one on the left.</Paragraph>
    <Paragraph position="8"> OK. Thank you.</Paragraph>
    <Paragraph position="9"> Dialogue i Show me the generic concept called &amp;quot;employee.&amp;quot; OK. &lt;system displays network&gt; \[ can't fit a new IC below it. Can you move it up? Yes. &lt;system displays network&gt; OK, now make an individual employee concept whose first name is &amp;quot;Sam&amp;quot; and whose last name is &amp;quot;Jones.&amp;quot; The  While still &amp;quot;task-oriented,&amp;quot; these dialogues illustrate phenomena characteristic of spontaneous conversation. That is, subdialogues correspond not only to subtasks (utterances (6)-(7) and (10)-(11)), but also to clarifications ((3)-(4)), debugging of task execution ((8)-(9)), and other types of topic switch and resumption. Furthermore, since these are extended discourses rather than unrelated question/answer exchanges, participants need to use the information provided by previous utterances. For example, (3) would be difficult to understand without the discourse context of (1) and (2). Finally, these dialogues illustrate the following of conversational conventions such as terminating dialogues (utterance (5)) and answering questions appropriately. For example, in response to (1), the clerk could have conveyed much the same information with &amp;quot;The departure location of train 537 is gate seven,&amp;quot; which would not have been as appropriate.</Paragraph>
    <Paragraph position="10"> To address these issues, we are developing a plan-based natural language system that incorporates knowledge of both task and discourse structure. In particular, we develop a new model of plan recognition that accounts for the recursive nature of plan suspensions and resumptions. Section 2 presents this model, followed in Section 3 by a brief description of the discourse analysis performed and the task and discourse interactions. Section 4 then traces the processing of Dialogue 1 in detail, and then this work is compared to previous work in Section 5.</Paragraph>
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