Mixed Initiative in Dialogue: An Investigation into Discourse Segmentation

Conversation between two people is usually of MIXED-INITIATIVE, with CONTROL over the conversation being transferred from one person to another. We apply a set of rules for the transfer of control to 4 sets of dialogues consisting of a total of 1862 turns. The application of the control rules lets us derive domain-independent discourse structures. The derived structures indicate that initiative plays a role in the structuring of discourse. In order to explore the relationship of control and initiative to discourse processes like centering, we analyze the distribution of four different classes of anaphora for two data sets. This distribution indicates that some control segments are hierarchically related to others. The analysis suggests that discourse participants often mutually agree to a change of topic. We also compared initiative in Task Oriented and Advice Giving dialogues and found that both allocation of control and the manner in which control is transferred is radically different for the two dialogue types. These differences can be explained in terms of collaborative planning principles. 1 I n t r o d u c t i o n Conversation between two people has a number of characteristics that have yet to be modeled adequately in human-computer dialogue. Conversation is BIDIRECTIONAL; there is a two way flow of information between participants. Information *This research was partially funded by ARO grants DAAG29-84-K-0061 and DAAL03-89-C0031PRI, DARPA grant N00014-85-K0018, and NSF grant MCS-82-19196 at the University of Pennsylvania, and by Hewlett Packard, U.K. is exchanged by MIXED-INITIATIVE. Each participant will, on occasion, take the conversational lead. Conversational partners not only respond to what others say, but feel free to volunteer information that is not requested and sometimes ask questions of their own[Nic76]. As INITIATIVE passes back and forth between the discourse participants, we say that CONTROL over the conversation gets transferred from one discourse participant to another. Why should we, as computational linguists, be interested in factors that contribute to the interactivity of a discourse? There are both theoretical and practical motivations. First, we wish to extend formal accounts of single utterances produced by single speakers to explain multi-participant, multi-utterance discourses[Po186, CP86]. Previous studies of the discourse structure of multiparticipant dialogues have often factored out the role of MIXED-INITIATIVE, by allocating control to one participant[Gro77, Coh84], or by assuming a passive listener[McK85, Coh87]. Since conversation is a collaborative process[CWG86, SSJ74], models of conversation can provide the basis for extending planning theories[GS90, CLNO90]. When the situation requires the negotiation of a collaborative plan, these theories must account for the interacting beliefs and intentions of multiple participants. ~,From a practical perspective, there is ample evidence that limited mixed-initiative has contributed to lack of system usability. Many researchers have noted that the absence of mixed-initiative gives rise to two problems with expert systems: They don't allow users to participate in the reasoning process, or to ask the questions they want answered[PHW82, Kid85, FL89]. In addition, question answering systems often fail to take account of the system's role as a conversational partner.