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<Paper uid="J88-3011">
  <Title>USER MODELS AND DISCOURSE MODELS: UNITED THEY STAND . . .</Title>
  <Section position="1" start_page="0" end_page="0" type="metho">
    <SectionTitle>
USER MODELS AND DISCOURSE MODELS:
UNITED THEY STAND . . .
</SectionTitle>
    <Paragraph position="0"> Opinions on the relationship between discourse models (DMs) and user models (UMs) are obviously influenced by preassumptions about their respective contents. As far as DMs are concerned, two divergent views have been expressed in the discussion published here: I. The DM contains only representations of the objects mentioned so far in the discourse (i.e., a mentioned-object memory--see Schuster, this issue). The term &amp;quot;object&amp;quot; will be used here in the broad sense of Schuster, thus also denoting events, properties, etc.</Paragraph>
  </Section>
  <Section position="2" start_page="0" end_page="0" type="metho">
    <SectionTitle>
2. The DM contains in addition
</SectionTitle>
    <Paragraph position="0"> a. a representation of the purpose underlying the segments of the dialog (i.e. a dialog purpose--see Grosz Sidner 1986, Chin, this issue).</Paragraph>
    <Paragraph position="1"> b. an attentional structure, which is a subset of the representations mentioned in (1) containing the currently focused objects which are ordered in a focus stack (Cohen, this issue; Chin, this issue, who requires only that the user must be familiar with these objects).</Paragraph>
    <Paragraph position="2"> Less disagreement seems to exist about the components of a UM. Generally, it is regarded as containing explicit representations of the system's assumptions about all relevant aspects of the user, i.e., assumptions about his/ her &amp;quot;objective situation&amp;quot; (e.g., marital status, number of children), as well as about his/her prior knowledge, goals, plans and false beliefs with respect to the domain of discourse. In order to meet Wahlster's personneldatabase counterexample, it must be further required that the user model be separable by the system from the rest of the system's knowledge.</Paragraph>
    <Paragraph position="3"> To discuss the relationship between DMs and UMs, a general belief, goal, and plan maintenance system (BGP-MS) will be presented here, the purpose of which is to store and update the beliefs, goals, and plans of both the system and an arbitrary number of other agents, including the system's current user. Specific subcomponents and subfunctions of this system hopefully capture the general consensus on what constitutes a discourse model and a user model, respectively.</Paragraph>
    <Paragraph position="4"> However, we will see that these subcomponents are strongly interwoven and that--apart from a few rarely occurring exceptions--the DM is part of the UM at least at the level of content. The question arises then, of course, whether it makes sense to separate these notions conceptually.</Paragraph>
    <Paragraph position="5"> The belief, goal, and plan maintenance system outlined here is being implemented (in a somewhat simplified form) in XTRA, a natural language access system to expert systems (Allgayer et al. 1988). A previous implementation was VIE-DPM (Kobsa 1985a,b). In the knowledge base of BGP-MS, the representation of the various types of (nested) beliefs and goals (Kobsa 1988) is separated into a number of hierarchically ordered partitions (see Figure 1). If it is shared knowledge between S and U that U possesses certain beliefs (knowledge), then this knowledge or these beliefs are represented in MB(UB). 1 MB(UW) contains those goals and plans of the user, MB(SB) those beliefs of the system, and MB(SW) those goals of the system for which the same holds true. &amp;quot;Private&amp;quot; beliefs of the system about the domain of discourse / about the user's beliefs / about the user's beliefs about the system's goals are represented in SB, SBUB, and SBUBSW, respectively. MB contains the mutual beliefs (knowledge) with respect to the domain, and MW the mutual goals and plans of S and U. The arrows between the partitions denote inheritance relationships.</Paragraph>
    <Paragraph position="6"> In the partitions of BGP-MS, the content of the individual beliefs, goals, and plans can be expressed through arbitrary representational structures (e.g., a KL-ONElike representation as used in XTRA). Various markers for non-belief and uncertainty can be added: For instance, in SBUB it can be expressed, among other Copyright 1988 by the Association for Computational Linguistics. Permission to copy without fee all or part of this material is granted provided that the copies are not made for direct commercial advantage and the CL reference and this copyright notice are included on the first page. To copy otherwise, or to republish, requires a fee and/or specific permission. 0362-613X/88/0100e-e$ 03.00 Computational Linguistics, Volume 14, Number 3, September 1988 91 Affred Kohsa User Models and Discourse Models: United they stand...</Paragraph>
    <Paragraph position="8"> in BGP-MS.</Paragraph>
    <Paragraph position="9"> things, that S is uncertain whether (or does not believe that) U knows some fact; and in MB(UB), that S is uncertain (or does not believe) that a belief of the user is mutually known.</Paragraph>
    <Paragraph position="10"> Where are the user model and the discourse model located in this architecture? The UM part of BGP-MS consists of all partitions except SB and SW, plus all representations in SB (and probably SW) in which an individual constant occurs denoting the user (the rest of SB corresponds to Sparck Jones's world model). The DM cannot be so easily identified. In the following sections I will discuss how the different functions of a discourse model as outlined above can be fulfilled by the proposed architecture.</Paragraph>
  </Section>
  <Section position="3" start_page="0" end_page="0" type="metho">
    <SectionTitle>
1. THE MENTIONED OBJECT MEMORY
</SectionTitle>
    <Paragraph position="0"> When an object is mentioned during the discourse, then mutual knowledge about its existence is usually established. Thus a representation of the object can be entered into MB (implying, for instance, that S can now use definite NPs to refer to these objects). Finer distinctions in what is now known can also be expressed: to take Chin's example, if S mentions a name unknown to U (i.e., the relationship between the name and its bearer is contained in SB only), then the existence of a person with this name can be entered into MB. In MB(UB) it can be represented that U does not know, and in bAB_(SB) that S does know the bearer of this name.</Paragraph>
    <Paragraph position="1"> All of the mentioned partitions are part of the user model. One might argue that the above architecture may completely cover the mentioned object memory function in an NL dialog system. However, three kinds of information are lost thereby (this defect also seems to apply partly to Schuster's model).</Paragraph>
    <Paragraph position="2"> a. The information that objects have been explicitly mentioned in the discourse: MB contains not only those objects which were explicitly mentioned in the discourse (and which are therefore mutually known by all dialog participants), but also representations of those objects whose existence is mutually known due to stereotypes (Rich 1988) or to inferences from the discourse. Sometimes, however, the system should possess information about whether or not some object had been explicitly mentioned, for example in order to increase coherency in its own dialog contributions (&amp;quot;As I/you said before... &amp;quot;) or to point out inconsistencies in dialog contributions of the user (&amp;quot;But previously you said...&amp;quot;).</Paragraph>
    <Paragraph position="3"> b. Information about the sequence (and thus recency) of objects' mention: This information is very important in NL systems, since the choice of various forms of anaphora depends on the degree of recency.</Paragraph>
    <Paragraph position="4"> c. hlformation about the linguistic structure of dialog contributions: Sometimes the system should also possess information about the wording or the syntactic structure of the user's and the system's previous dialog contributions (i.e., information on how objects have been mentioned). This information can be exploited by the system for reiterating a description in its own dialog contributions or for avoiding reiteration, for instance.</Paragraph>
    <Paragraph position="5"> In XTRA, two additional knowledge bases have been introduced which serve the above-mentioned functions, among others: the FSS knowledge base and the so-called Linguistic Dialog Memory. The FSS (Allgayer and Reddig 1986) represents the functional semantic structure of both the user's and the system's dialog contributions. FSS contents can also be linked to the linguistic surface forms (NPs, PPs, etc.) which caused their creation (in the case of user input) or became their linguistic realizations (in the case of system dialog contributions). The dialog memory, among other things, records the objects that have been mentioned during the discourse, and, in its dialog sequence part, the sequence of the objects' mention.</Paragraph>
    <Paragraph position="6"> In general, all system knowledge about what objects have been mentioned in the on-going discourse, in what  order they were mentioned, and how they were described (i.e. all parts of the dialog memory) are regarded as being part of MB, and hence part of the user model. This is necessarily so, since, by definition, MB contains all knowledge that is shared by system and user. And only if knowledge about the previous discourse is shared between both participants can it be safely employed in the generation of dialog contributions. (For example, an anaphor generated by the system will probably fail to fulfill its referential function if only the system, but not the user, believes that its intended referent has been mentioned just recently. Hence the system should check whether its records in the dialog sequence memory are shared by the user.) In cases of communicative failure, however, there exists system discourse knowledge that is not shared by the user, and thus not part of MB and the user model. In such cases, entries are made into SB instead, and thereby form part of the system's knowledge only. For instance, when S assumes that U does not remember what has been said (see Wahlster's hastily-presentednames example), the FSS descriptions and the representations of their referents in the dialog memory can be entered in SB instead of MB, and thus do not form part of the user model. In addition, however, all sorts of uncertain assumptions about what the user has or has not, in fact, kept in mind can be expressed in SBUB or MB(UB), i.e., in the user model. (For example, the system can note in the user model that the user probably remembered the first two but not the subsequent names.) For simplicity, however, neither of the described cases of communicative failure is dealt with in XTRA, and for implementational reasons the FSS part of MB forms a separate partition.</Paragraph>
    <Paragraph position="7"> 2A. THE DIALOG PURPOSE Here the problem arises in research on NL dialog systems as to whether or not one should regard the intentional structure of individual utterances (Cohen, this issue) or dialog segments (Grosz Sidner 1986) as being independent of the dialog setting and the dialog participants. To put it in a more provocative way, do dialog constituents or dialog participants have an intentional structure? In my view, the essence of problem-solving dialog lies in the recognition of the dialog participants' goals and plans, and in the construction of mutually known goals and plans. Dialog contributions of the dialog partner serve as a more-or-less helpful aid in this process (Pollack et al. (1982) present transcripts in which dialog contributions of clients with misconceptions even impair the recognition of their actual goals). Apart from that, no intentional character pertains to dialog constituents that is independent of the dialog and situational context and of the current (beliefs about) goals and plans.</Paragraph>
    <Paragraph position="8"> In the BGP-MS philosophy, (beliefs about) goals and plans are contained in those partitions whose labels include a W. A user's dialog contribution is first represented by FSS structures in MB. If user plans or goals can be inferred by S, they are represented in MB(UW), or in MW if there is mutual knowledge that they have been accepted by S. Conversely, when S gradually communicates its plans or goals to U, the corresponding representation structures are transferred from SW to MB(SW), and finally hopefully to MW. Thus any sort of intentional structure is part of the user model.</Paragraph>
    <Paragraph position="9"> 2B. THE ATTENTIONAL STRUCTURE I agree with Chin (this issue) that the attentional structure is also not a context-independent characteristic of discourse (although Chin's notion of attentional structure seems to be broader than mine). Only mutually known objects can be in focus. In XTRA, focus is expressed by focus values in the dialog memory which, logically, can only be applied to representation structures in MB and MW and therefore form part of the user model.</Paragraph>
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