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<Paper uid="J93-3003">
  <Title>Empirical Studies on the Disambiguation of Cue Phrases</Title>
  <Section position="2" start_page="0" end_page="502" type="intro">
    <SectionTitle>
1. Introduction
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    <Paragraph position="0"> Cue phrases, words and phrases that directly signal the structure of a discourse, have been variously termed clue words, discourse markers, discourse connectives, and discourse particles in the computational linguistic and conversational analysis literature. These include items such as now, which marks the introduction of a new subtopic or return to a previous one; well, which indicates a response to previous material or an explanatory comment; incidentally, by the way, and that reminds me, which indicate the beginning of a digression; and anyway and in any case, which indicate a return from a digression. The recognition and appropriate generation of cue phrases is of particular interest to research in discourse structure. The structural information conveyed by these phrases is crucial to many tasks, such as anaphora resolution (Grosz 1977; Grosz and Sidner 1986; Reichman 1985), the inference of speaker intention and the recognition of speaker plans (Grosz and Sidner 1986; Sidner 1985; Litman and Allen 1987), and the generation of explanations and other text (Zuckerman and Pearl 1986).</Paragraph>
    <Paragraph position="1"> Despite the crucial role that cue phrases play in theories of discourse and their implementation, however, many questions about how cue phrases are identified and defined remain to be examined. In particular, the question of cue phrase polysemy has yet to receive a satisfactory solution. Each lexical item that has one or more discourse * 600 Mountain Avenue, Murray Hill, NJ 07974.</Paragraph>
    <Paragraph position="2"> (D 1993 Association for Computational Linguistics Computational Linguistics Volume 19, Number 3 senses also has one or more alternate, sentential senses, which make a semantic contribution to the interpretation of an utterance. So, sententially, now may be used as a temporal adverbial, incidentally may also function as an adverbial, and well may be used with its adverbial or attributive meanings. Distinguishing between whether a discourse or a sentential usage is meant is obviously critical to the interpretation of discourse.</Paragraph>
    <Paragraph position="3"> Consider the cue phrase now. Roughly, the sentential or deictic use of now makes reference to a span of time that minimally includes the utterance time. This time span may include little more than moment of utterance, as in Example 1, or it may be of indeterminate length, as in Example 2.</Paragraph>
    <Paragraph position="4"> Example 1 Fred: Yeah I think we'll look that up and possibly uh after one of your breaks Harry. Harry: OK we'll take one now. Just hang on Bill and we'll be right back with you. Example 2 Harry: You know I see more coupons now than I've ever seen before and I'll bet you have too.</Paragraph>
    <Paragraph position="5"> These examples are taken from a radio call-in program, &amp;quot;The Harry Gross Show: Speaking of Your Money&amp;quot; (Pollack, Hirschberg, and Webber 1982), which we will refer to as (HG82). This corpus will be described in more detail in Section 4. In contrast, the discourse use of now signals a return to a previous topic, as in the two examples of now in Example 3 (HG82), or introduces a subtopic, as in Example 4 (HG82).</Paragraph>
    <Paragraph position="6"> Example 3 Harry: Fred whatta you have to say about this IRA problem? Fred: OK. You see now unfortunately Harry as we alluded to earlier when there is a distribution from an IRA that is taxable ... discussion of caller's beneficiary status... Now the five thousand that you're alluding to uh of the---Example 4 Doris: I have a couple quick questions about the income tax. The first one is my husband is retired and on social security and in '81 he ... few odd jobs for a friend uh around the property and uh he was reimbursed for that to the tune of about $640. Now where would he where would we put that on the form? Example 5 nicely illustrates both the discourse and sentential uses of now in a single utterance.</Paragraph>
    <Paragraph position="7"> Example 5 Now now that we have all been welcomed here it's time to get on with the business of the conference.</Paragraph>
    <Paragraph position="8"> In particular, the first now illustrates a discourse usage, and the second a sentential usage. This example is taken from a keynote address given by Ronald Brachman to the First International Conference on Expert Database Systems in 1986. We will refer to this corpus as RJB86. The corpus will be described in more detail in Section 5.  Julia Hirschberg and Diane Litman Disambiguation of Cue Phrases While the distinction between discourse and sentential usages sometimes seems quite clear from context, in many cases it is not. From the text alone, Example 6 (RJB86) is potentially ambiguous between a temporal reading of now and a discourse interpretation.</Paragraph>
    <Paragraph position="9"> Example 6 Now in AI our approach is to look at a knowledge base as a set of symbolic items that represent something.</Paragraph>
    <Paragraph position="10"> On the temporal reading, Example 6 would convey that 'at this moment the AI approach to knowledge bases has changed;' on the discourse reading, now simply initiates the topic of 'the AI approach to knowledge bases.' In this paper, we address the problem of disambiguating cue phrases in both text and speech. We present results of several studies of cue phrase usage in corpora of recorded, transcribed speech, in which we examined text-based and prosodic features to find which best predicted the discourse/sentential distinction. Based on these analyses, we present an intonational model for cue phrase disambiguation in speech, based on prosodic phrasing and pitch accent. We associate this h~odel with features identifiable from text analysis, principally orthography and part of speech, that can be automatically extracted from large corpora. On a practical level, this association permits the application of our findings to the identification and appropriate generation of cue phrases in synthetic speech. On a more theoretical level, our findings provide support for theories of discourse that rely upon the feasibility of cue phrase disambiguation to support the identification of discourse structure. Our results provide empirical evidence suggesting how hearers and readers may distinguish between discourse and sentential uses of cue phrases. More generally, our findings can be seen as a case study demonstrating the importance of intonational information to language understanding and generation.</Paragraph>
    <Paragraph position="11"> In Section 2 we review previous work on cue phrases and discuss the general problem of distinguishing between discourse and sentential uses. In Section 3 we introduce the theory of English intonation adopted for our prosodic analysis (Pierrehumbert 1980; Beckman and Pierrehumbert 1986). In Section 4 we present our initial empirical studies, which focus on the analysis of the cue phrases now and well in multispeaker spontaneous speech. In Section 5 we demonstrate that these results generalize to other cue phrases, presenting results of a larger and more comprehensive study: an examination of all cue phrases produced by a single speaker in a 75-minute presentation. Finally, in Section 6 we discuss the theoretical and practical applications of our findings.</Paragraph>
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