A Plan Recognition Model for Clarification Subdialogues

A Plan Recognition Model for Clarification Subdialogues Diane J. Litman and James F. Allen Department of Computer Science University of Rochester, Rochester, NY 14627 Abstract 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. 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. 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. accounts for the debugging subdialogues they discussed, as well as other forms of clarification and topic shi~. 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. 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]. 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. Where …

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