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<Paper uid="J95-3003">
  <Title>Collaborating on Referring Expressions</Title>
  <Section position="2" start_page="0" end_page="353" type="abstr">
    <SectionTitle>
1. Introduction
</SectionTitle>
    <Paragraph position="0"> People are goal-oriented and can plan courses of actions to achieve their goals. But sometimes they might lack the knowledge needed to formulate a plan of action, or some of the actions that they plan might depend on coordinating their activity with other agents. How do they cope? One way is to work together, or collaborate, in formulating a plan of action with other people who are involved in the actions or who know the relevant information.</Paragraph>
    <Paragraph position="1"> Even the apparently simple linguistic task of referring, in an utterance, to some object or idea can involve exactly this kind of activity: a collaboration between the speaker and the hearer. The speaker has the goal of the hearer identifying the object that the speaker has in mind. The speaker attempts to achieve this goal by constructing a description of the object that she thinks will enable the hearer to identify it. But since the speaker and the hearer will inevitably have different beliefs about the world, the hearer might not be able to identify the object. Often, when the hearer cannot do so, the speaker and hearer collaborate in making a new referring expression that accomplishes the goal.</Paragraph>
    <Paragraph position="2"> This paper presents a computational model of how a conversational participant collaborates in making a referring action successful. We use as our basis the model proposed by Clark and Wilkes-Gibbs (1986), which gives a descriptive account of the conversational moves that participants make when collaborating upon a referring expression. We cast their work into a model based on the planning paradigm.</Paragraph>
    <Paragraph position="3"> We propose that referring expressions can be represented by plan derivations, and that plan construction and plan inference can be used to generate and understand them. Not only does this approach allow the processes of building referring expres- null * Department of Computer Science, Rochester, New York 14627. E-mail: heeman@cs.rochester.edu. t Department of Computer Science, Toronto, Canada, M5S 1A4. E-maih gh@cs.toronto.edu. (c) 1995 Association for Computational Linguistics  Computational Linguistics Volume 21, Number 3 sions and identifying their referents to be captured in the planning paradigm, but it also allows us to use the planning paradigm to account for how participants clarify a referring expression. In this case, we use meta-actions that encode how a plan derivation corresponding to a referring expression can be reasoned about and manipulated.</Paragraph>
    <Paragraph position="4"> To complete the picture, we also need to account for the fact that the conversants are collaborating. We propose that the agents are in a mental state that includes not only an intention to achieve the goal of the collaborative activity but also a plan that the participants are currently considering. In the case of referring, this will be the plan derivation that corresponds to the referring expression. This plan is in the common ground of the participants, and we propose rules that are sanctioned by the mental state both for accepting plans that clarify the current plan, and for adopting goals to do likewise. The acceptance of a clarification results in the current plan being updated. So, it is these rules that specify how plan inference and plan construction affect and are affected by the mental state of the agent. Thus, the mental state, together with the rules, provides the link between these two processes. An important consequence of our proposal is that the current plan need not allow the successful achievement of the goal. Likewise, the clarifications that agents propose need not result in a successful plan in order for them to be accepted.</Paragraph>
    <Paragraph position="5"> As can be seen, our approach consists of two tiers. The first tier is the planning component, which accounts for how utterances are both understood and generated.</Paragraph>
    <Paragraph position="6"> Using the planning paradigm has several advantages: it allows both tasks to be captured in a single paradigm that is used for modeling general intelligent behavior; it allows more of the content of an utterance to be accounted for by a uniform process; and only a single knowledge source for referring expressions is needed instead of having this knowledge embedded in special algorithms for each task. The second tier accounts for the collaborative behavior of the agents: how they adopt goals and co-ordinate their activity. It provides the link between the mental state of the agent and the planning processes.</Paragraph>
    <Paragraph position="7"> In accounting for how agents collaborate in making a referring action, our work aims to make the following contributions to the field. First, although much work has been done on how agents request clarifications, or respond to such requests, little attention has been paid to the collaborative aspects of clarification discourse. Our work attempts a plan-based formalization of what linguistic collaboration is, both in terms of the goals and intentions that underlie it and the surface speech acts that result from it. Second, we address the act of referring and show how it can be better accounted for by the planning paradigm. Third, previous plan-based linguistic research has concentrated on either construction or understanding of utterances, but not both.</Paragraph>
    <Paragraph position="8"> By doing both, we will give our work generality in the direction of a complete model of the collaborative process. Finally, by using Clark and Wilkes-Gibbs's model as a basis for our work, we aim not only to add support to their model, but also to gain a much richer understanding of the subject.</Paragraph>
    <Paragraph position="9"> In order to address the problem that we have set out, we have limited the scope of our work. First, we look at referring expressions in isolation, rather than as part of a larger speech act. Second, we assume that agents have mutual knowledge of the mechanisms of referring expressions and collaboration. Third, we deal with objects that both the speaker and hearer know of, though they might have different beliefs about what propositions hold for these objects. Fourth, as the input and the output to our system, we use representations of surface speech actions, not natural language strings. Finally, although belief revision is an important part of how agents collaborate, we do not explicitly address this.</Paragraph>
    <Paragraph position="10">  Peter A. Heeman and Graeme Hirst Collaborating on Referring Expressions 2. Referring as a Collaborative Process  Clark and Wilkes-Gibbs (1986) investigated how participants in a conversation collaborate in making a referring action successful. They conducted experiments in which participants had to refer to objects--tangram patterns--that are difficult to describe. They found that typically the participant trying to describe a tangram pattern would present an initial referring expression. The other participant would then pass judgment on it, either accepting it, rejecting it, or postponing his decision. If it was rejected or the decision postponed, then one participant or the other would refashion the referring expression. This would take the form of either repairing the expression by correcting speech errors, expanding it by adding further qualifications, or replacing the original expression with a new expression. The referring expression that results from this is then judged, and the process continues until the referring expression is acceptable enough to the participants for current purposes. This final expression is contributed to the participants' common ground.</Paragraph>
    <Paragraph position="11"> Below are two excerpts from Clark and Wilkes-Gibbs's experiments that illustrate the acceptance process.</Paragraph>
    <Paragraph position="12"> (2.1) A: 1 Um, third one is the guy reading with, holding his book to the left.</Paragraph>
    <Paragraph position="13"> B- 2 Okay, kind of standing up? A: 3 Yeah.</Paragraph>
    <Paragraph position="14"> B&amp;quot; 4 Okay.</Paragraph>
    <Paragraph position="15"> In this dialog, person A makes an initial presentation in line 1. Person B postpones his decision in line 2 by voicing a tentative &amp;quot;okay,&amp;quot; and then proceeds to refashion the referring expression, the result being &amp;quot;the guy reading, holding his book to the left, kind of standing up.&amp;quot; A accepts the new expression in line 3, and B signals his acceptance in line 4.</Paragraph>
    <Paragraph position="16"> (2.2) A: 1 Okay, and the next one is the person that looks like they're carrying something and it's sticking out to the left. It looks like a hat that's upside down.</Paragraph>
    <Paragraph position="17"> B: 2 The guy that's pointing to the left again? A&amp;quot; 3 Yeah, pointing to the left, that's it! (laughs) B: 4 Okay.</Paragraph>
    <Paragraph position="18"> In the second dialog, B implicitly rejects A's initial presentation by replacing it with a new referring expression in line 2, &amp;quot;the guy that's pointing to the left again.&amp;quot; A then accepts the refashioned referring expression in line 3.</Paragraph>
    <Paragraph position="19"> Below, we give an algorithmic interpretation of Clark and Wilkes-Gibbs's collaborative model, where present, judge, and refashion are the conversational moves that the participants make, and ref, re, and judgment are variables that represent the referent, the current referring expression, and its judgment, respectively. (Since the conversational moves update the referring expression and its judgment, they are presented as functions.)</Paragraph>
    <Paragraph position="21"> The algorithm illustrates how the collaborative activity progresses by the participants judging and refashioning the previously proposed referring expression. 1 In fact, we can see that the state of the process is characterized by the current referring expression, re, and the judgment of it, judgment, and that this state must be part of the common ground of the participants. The algorithm also illustrates how the model of Clark and Wilkes-Gibbs minimizes the distinction between the roles of the person who initiated the referring expression and the person who is trying to identify it. Both have the same moves available to them, for either can judge the description and either can refashion it. Neither is controlling the dialog, they are simply collaborating.</Paragraph>
    <Paragraph position="22"> In later work, Clark and Schaefer (1989) propose that &amp;quot;each part of the acceptance phase is itself a contribution&amp;quot; (p. 269), and the acceptance of these contributions depends on whether the hearer &amp;quot;believes he is understanding well enough for current purposes&amp;quot; (p. 267). Although Clark and Schaefer use the term contribution with respect to the discourse, rather than the collaborative effort of referring, their proposal is still relevant here: judgments and refashionings are contributions to the collaborative effort and are subjected to an acceptance process, with the result being that once they are accepted, the state of the collaborative activity is updated. So, what constitutes grounds for accepting a judgment or clarification? In order to be consistent with Clark and Wilkes-Gibbs's model, we can see that if one agent finds the current referring expression problematic, the other must accept that judgment. Likewise, if one agent proposes a referring expression, through a refashioning, the other must accept the refashioning.</Paragraph>
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