A Pragmatics-Based Approach to Understanding
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A PRAGMATIC~BASED APPROACH TO UNDERSTANDING INTERS~NTENTIAL ~LIPSI~ Sandra Car berry Department of Computer and Information Science University of Delaware Nevark, Delaware 19715, U3A ABSTRACT S~A~R-I : "The Korean Jet shot down by the Soviets was a spy plane." IntersententAal eAlipti caA utterances occur frequently in information-seeking dielogues. This FI: "With 269 people on board?"~ paper presents a pragmatics-based framework for F2: "With infrared cameras on board?" interpreting such utterances, ~ncluding identAfi- cation of the spoa~r' s discourse ~oel in employ- Previous research on ellipsis has neglected to ing the fra~ent. We claim that the advantage of address the speaker's discourse Eoals in employing this approach is its reliance upon pragmatic the frasment but reel understanding requires that information, including discourse content and these be identified (Mann, Moore, and Levin, 1977) conversational goals, rather than upon precise (Webber, PoZlack, and Hirschberg, 1982). representations of the preceding utterance alone. In this paper, we investlgate a framework for interpreting Intersententlal ellipsis that occurs INTRODOCTION in task-orlented dialogues. This framework includes: The fraRmentary utterances that are common in communication between humans also occur in man- [1] a context mechanism (Carberry, 1983) that Nachi~e OOmmUlLCcation. Humans perslat in using builds the information-seeker, s underlying abbreviated statements and queries, even in the plan as the dialogue progresses and differen- presence o/ explicit and repeated instructions to tiates be~een local and global contexts. adhere to syntactically and semantically complete sentences (Carbonell, 1983) • Thus a robust [2] a discourse component that controls the natural langua@e interface must handle ellipsis. interpretation of ellipsis based upon discourse goal expectations ~eaned from the We have studied one class of elliptical dial o@ue ; this component "understands" utterances, Intersentential fragments, in the con- ellipsis by identifying the" discourse goal text of an Information-seeklng dialogue. As noted which the speaker is pursuing by employing by Allen(1980), such utterances differ from other the elliptical fragment, and by determining forms of ellipsis in that interpretation often how the frasment should be interpreted rela- depends more heavily upon the speaker's inferred tive to that goal. underlying task-related plan than upon preceding syntactic forms. For example, the fcllowlng [3] an analysis component that suggests possible elliptical fra@ment can only be interpreted within associations o£ an elliptical fragment with the context of the speaker's goal as communicated aspects of the inferred plan for the in the first utterance: information-seeker. [EX1 ] aT want to cash this check. [4] an evaluation component which, 51yen multiple Smell bills only. * possible associations o£ an elliptical frag- ment with aspects of the information-seeker,s Furthermore, intersententiel fragments are often underlying plan, selects that ansociation employed to communicate discourse 8oals, such as most appropriate to the discourse context and expressing doubt, which a syntactically complete believed to be intended by the speaker. form of the same utterance may not convey as effectively. In the following alternative responses to the initial statement by SPEAKER-I, INTERPRETATION OF INTERS~qTENTIAL ~LLTPSIS F1 expresses doubt regarding the proposition seated by 3PEAEZB-I whereas F2 merely asks about As Ltlustrated by [EX1], intersententiel the jet's contents. eA1ipticaA fra@ments cannot be fully understood in and of themselves. Therefore a strate8~ for • This work has been partially supported by a interpreting suc~ fra@ments must rely on knowledge grant from the National 3cAence Foundation, XST- obtained frcl sources other than the fragment 8311~00, and a subeontraot from Bolt Beranek and itself. Three possibilities exist: the syntactic Newmm'l Inc. of a grant flwm the Nationa~ ScAence Foundation, T~T-8~19162 ee Ta.~n fr'~ Flowers and Dyer(198~) 188 form ar precedlug utterances, the seaantlo tlon strategy vurke well. In more complex representation of preceding utterances, and expec- domains, it Is necessary to identify the particu- tations gleaned from understanding the preceding lar aspect of the speaker's overall task-related disQourse. plan addressed by the clliptlcal frasment in order to interpret It properly. More recently, Litman The first two strategies are exemplified by and Allen(198q) have extended Allen's model to a the work oC Carbosoll and Hayes(1983), Hendrlx, hierarchy of task-plans and meta-plans. Litman is Sacerdot¢, and Sloc,~( 1976), Waltz( 1978), and currently studying the interpretation of ellipti- Velschedel and 3ondhelmer( 1982 ). Several limita- cal frasments within this enhanced framework. tions exist in these approaches, includiug an ina- bilit 7 to handle utterances that rely upon an In addition to the syntactic, semantic, and assumed communication of the underlying task and plan-based strategies, a few other heuristics have difficulty in resolving ambiguity ="oug multiple been utilized. Carbusoll(1983) uses discourse interpretations. Consider the following two expectation rules that suggest a set of expected dislo~e sequences: user utterances and relate elliptical f~a~ents to these expected patterns. For example, if the sys- SPEAE~R: "I want to take a bus. t~a asks the user whether a particular value The cost?" should be used an the filler o£ a slot in a case frane, the system then expects the user's utter- SPEAKER: "I want to purchase a bus. ance to contain a confirmation or disson~Irmatlon The cost?" pattern, a different filler for the slot, a com- parative pattern such as "too hard", and so forth. Zf a semantic strategy is employed, the case frame Although these rules use expectations about how representation for "bus" may have a "cost of bus" the speaker m~ght respond, they seem to have llt- and a "cost of bus ticket" slot; a~hlgulty arises tle to do with the expected discourse goals of the regardlug to which slot the elliptical fr~sment speaker. "The cost?" refers. Althou~ one might suggest extensions far handling this fra~ent, a semantic Real understanding consists sot only of strategy alone does not provide an adequate frame~- resognlzAr~ the particular surface-request or wurk for Interpreting intersentential ellipsis. surface-lnform, but also of inferring what the speaker wants to accomplish and the relationship The third potential strategy utilizes a model of each utterance to this task. Interpretation of c~ the information-seeker's inferred tank-related ellipsis based upon the speaker's inferred under~ plan and discourse ~oals. The power of this lying task-related plan and discourse Eoals facil- approach is its reliance upon pragmatic informa- itates a richer interpretation of elliptical tion, including discourse content and converse- utterances. tiona~ goals, rather than upon precise representa- tions of the preceding utterances alone. REQUISITE KNCWLEDG E Allen(1980) was the first to relate ellipsis processlug to the domain-dependent plan underlying A speaker can felicitously employ intersen- a speaker's utterance. Allen views the speaker's tentlal ellipsis only Lf he believes his utterance utterance as part of a plan which the speaker has will be properly understood. The motivation for constructed and is executlug to accomplish his this work is the hypothesis that speaker and overall task-related goals. To interpret ellipti- hearer mutually believe that certain knowledge has cal fragments, Allen first constructs a set of been acquired during the course of the dialogue possible surface speech act representations for and that this factual knowledge along with other the elliptical fragment, limited by syntactic processing knowledge will be used to deduce the clues appearing within the fragment. The task- speaker,s intentions. We claim that the requisite related ~oals which the speaker might pursue form factual knowledge includes the speaker,s inferred a set o1" expectations, and Allen attempts to infer task-related plan, the speaker's inferred beliefs, the speaker's ~al-related plan which resulted in and the anticipated discourse Eoala of the execution of the observed utterance. A part of speaker; We claim that the requisite processing this inference process involves determining which knowledge includes plan recognltlon strategies and of the partially constructed plans connecting focuslng techniques. expectations (goals) and obeerved utterance are 'reasonable given the knovled~ and mutual beliefs 1. Task-Related Plan of the speaker and hearer. Allen selects the sur- face speech act which produced the most reasonable In a cooperative information-seeking dAelo- inferred plan as the correct interpretation. gue, the ln~ormation-provider is expected to infer the ir~ors~ation-seeker, s underlying task-related Allen notes that the speaker's fragment must plan an the dialogue pro~-eases. At any point An identif7 the subg~als which the spea~er is pursu- the dialo~e, ZS (the information-seeker) believes Lug, but claims that in very restricted dmaains, that soae subset of this plan has been coemunA- identifying the speaker's overall ~ from the mated to IP (the in~ormation-provider); therefore utterance ls sufficient to identify the appropri- Y~ feeAa Juatl.rled in ~ormuAating utterances under ate response in terms of the obstacles present in the assumption that IP will use this inferred task such a plan. For his restricted do~aln involving model to interpret utterances, includIDg elliptl- train arrivals and departures, Allen's Interprets- eLL frasmente. 189 An example will illustrate the importance of regarding IS at the time the elliptical fragment IS's inferred task-related plan in interpreting is uttered. In the latter case, IP believes it As ellipsis. In the following, IS is conslderi~ mutually believed that IS already knows IP' s purchase of a home mentioned earlier in the dialo- beliefs regarcling when C/~O0 meets, so a request ~ue: for that informatlon is not felicitous and a dif- ferent intention or discourse goal is attributed IS: "What elementary school do children to L~.