Discourse Pragmatics and Ellipsis Resolution in Task-Oriented Natural Language Interfaces

This paper reviews discourse phenomena that occur frequently in task.oriented man.machine dialogs, reporting on a~n empirical study that demonstrates the necessity of handling ellipsis, anaphora, extragrammaticality, inter-sentential metalanguage, and other abbreviatory devices in order to achieve convivial user interaction. Invariably, users prefer to generate terse or fragmentary utterances instead of longer, more complete "standalone" expressions, even when given clear instructions tO the contrary. The XCALIBUR exbert system interface is designed to meet these needs, including generalized ellipsis resolution by means of a rule-based caseframe method superior tO previous semantic grammar approaches. 1. A Summary of Task-Or iented Discourse Phenomena Natural language discourse exhibits several intriguing phenomena that defy definitive linguistic analysis and general computational solutions. However, some progress has been made in developing tractable computational solutions to simplified version of phenomena such as ellipsis and anaphora resolution [20, 10, 211. This paper reviews discourse phenomena that arise ~n task.oriented dialogs with responsive agents (such as expert systems, rather than purely passive data base query systems), outlines the results of an empirical study, and presents our method for handling generalized ellipsis resolution in the XCALIBUR expert system interface. With the exception of intersentential metalanguage, and to a lesser degree extragrammaticality, the significance of the phenomena listed below have long been recognized and documented in the computational linguistics literature. • Anaphora -Interactive task-oriented dialogs invite the use of anaphora, much more so than simpler data base query situations. • Def in i te noun phrases -As Grosz [6] noted, resolving the referent of defimte noun phrases requires an understanding of the planning structure underlying cooperative discourse, • El l ipsis -. Sentential level ellipsis has long been recognized as ubiquitous in discourse. However, semantic ellipsis, where ellipsed information is manifest not as syntactically incomplete structures, but as semantically incomplete propositions, is also an important phenomenon. The ellipsis resolution method presented later in this paper addresses both kinds of ellipsis. • Extragrammatical utterances -Interjections, dropped articles, false starts, misspellings, and other forms of grammatical deviance abound in our data (as explained in the following section). Developing robust parsing techniques that tolerate errors has been the focus of our earlier investigations [2, 9. 7] and remains high among our priorities. Other investigations on error-tolerant parsing incJude [13, 22]. • Meta. l inc ju is t ic u t te rances -Intra-sentential metalanguage has been investigated to some degree [18, 12J, but its more common inter-sententiai counterpart has received little attention [4}. However, utterances about other utterances (e,g,, corrections of previous commands, such as "1 meant to type X instead" or "1 should have said ...") are not infrequent in our dialogs, and we are making an initial stab at this problem [8}. Note that it is a cognitively less demanding task for a user to correct a previous utterance than to repeat an explicit sequence of commands (or worse yet, to detect and undo explicitly each and every unwanted consequence of a mistaken command). • i nd i rec t speech acts -Occasionally users will resort tO indirect speech acts[19. 16, 1], especially in connection with inter.sentential metalanguage or by stating a desired state of affairs and expecting the system tO supply the sequence of actions necessary to achieve that state. In our prior work we have focused on extr~grammaticality and inter.sentential metalanguage. In this paper we report on an empLrical study of discourse phenomena to a s~mulated interface and on our work on generalized elhpsis resolutLon in the context of the XCALIBUR project, 2. An Empirical Study The necessity to handle most of the discourse phenomena listed in the preceding section was underscored by an empirical study we conducted to ascertain the most pressing needs of natural language interfaces in interactive apl~lications, The initial objective of this study was to circumscribe the natural language interface task by attempting to instruct users of a simulated interface not to employ different discourse devices or difficult linguistic constructs. In essence, we wanted to determine whether untrained users would be able to interact as instructed (for instance avoiding all anaphoric referents), and, if so, whether they would still find the interface convivial given our artificial constraints. The basic experimental set-up consisted of two remotely located terminals linked to each other and a transaction log file