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<Paper uid="C00-2151">
  <Title>An Experiment On Incremental Analysis Using Robust Parsing Techniques</Title>
  <Section position="3" start_page="0" end_page="1026" type="intro">
    <SectionTitle>
1 Introduction
</SectionTitle>
    <Paragraph position="0"> Natural language utterances usually unfold over time, i. e., both listening and reading are carried out in an incremental left-to-right manner. Modeling a similar type of behaviour in computer-based solutions is a challenging aim particularly interesting task for a number of quite different reasons that are most relevant in the context of spoken language systems: * Without any external signals about the end of an utterance, incremental analysis is the only means to segment the incoming stream of speech input.</Paragraph>
    <Paragraph position="1"> * An incremental analysis mode provides for a lnore natural (mixed-initiative) dialogue behaviour because partial results are already available well before the end of an utterance.</Paragraph>
    <Paragraph position="2"> * Parsing may already take place in concurrency to sentence production. Therefore the speaking time becomes available as computing time.</Paragraph>
    <Paragraph position="3"> * Dynmnic expectations about the upcoming parts of the utterance might be derived right in time to provide guiding hints for other processing components, e. g., predictions about likely word forms for a speech recognizer (Hauenstein and Weber, 1994).</Paragraph>
    <Paragraph position="4"> In principle, two alternative strategies can be pursued when designing all incremental parsing proce- null dure: 1. To keel) open all necessary structural hypotheses required to accomodate every possible continuation of an utterance. This is tile strategy usually adopted in an incremental chart parser (WirSn, 1992).</Paragraph>
    <Paragraph position="5"> 2. To commit to one or a limited number of interpretations where (a) either this commitment is made rather early and a mechanisnl for partial reanalysis is provided (Lombardo, 1992) or (b) it is delayed until sufficient information is eventually available to take an ultimate decision (Marcus, 1987).</Paragraph>
    <Paragraph position="6">  The apparent efficiency of human language understanding is usually attributed to an early commitment strategy.</Paragraph>
    <Paragraph position="7"> Our approach, in fact, represents an attempt to combine these two strategies: On the one hand, it keeps many of the available building blocks for the initial part of an utterance and passes an ('updated) search space to the following processing step. Additionally, the optimal structural description for the data already available is determined. This not only makes an actual interpretation (together with expectations for possible continuations) available to subsequent processing components, but also opens up the possibility to use this information to effectively constrain the set of new structural hypotheses. Determining the optimal interpretation for a yet incomplete sentence, however, requires a parsing approach robust enough to analyse an arbitrary sentencc prefix into a meaningful structure. Therefore two closely related questions need to be raised:  1. Can the necessary degree of robustness be achieved? 2. Is the information contained in the currently optimal structure useful to guide the subsequent analysis?  To answer these questions and to estimate the potential for search space reductions against a 1)ossible loss of accuracy, a series of experiments has been conducted. Sections 2 and 3 introduce and motivate the framework of the exl)eriments. Section 4 describes a number of heuristics for re-use of previous solutions and Section 5 1)resents the results.</Paragraph>
  </Section>
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