Collaborative Plans for Group Activities

The original formulation of SharedPlans [Grosz and Sidner, 1990] was developed to provide a model of collaborative planning in which it was not necessary for one agent to have intentions toward an act of a different agent. This formulation provided for two agents to coordinate their activities without introducing any notion of jointly held intentions (or, 'we-intentions'). However, it only treated activities that directly decomposed into single agents actions. In this paper we provide a revised and expanded version of SharedPlans that accommodates actions involving groups of agents as well as complex actions that decompose into multi-agent actions. The new definitions also allow for contracting out certain actions, and provide a model with the features required in Bratman's account of shared cooperative activity [Bratman, 1992]. A reformulation of the model of individual plans that meshes with the definition of SharedPlans is also provided. 1 Introduction Collaboration in planning and acting is an essential ingredient of multi-agent cooperative problem solving. In this paper we present a model of collaborative planning that supports cooperative problem solving by teams consisting of humans and computer systems. The model deals more completely with collaboration than previous theories did in meeting two criteria. First, collabora-tive planning and activity cannot be analyzed simply in terms of the plans of individual agents, but require an integrated treatment of the beliefs and intentions of the different collaborating agents. Second, collaborative planning is a refinement process; a partial plan description is modified over the course of planning by the mul-* We thank Joyce Friedman for many provoking questions, and Karen Lochbaum for the same and for helpful comments on many drafts. tiple collaborating agents. This model grew out of an attempt to provide an adequate treatment of the intentional component of discourse structure [Grosz and Sidner, 1986]. However, many multi-agent situations require that agents have an ability to plan and act together ; merely avoiding conflicting actions or situations is not sufficient. Thus, the model is applicable not only to natural language processing, but also to the general problem of the design of computer-based collaborating agents. The original formulation of the SharedPlan model of collaborative planning [Grosz and Sidner, 1990] extended Pollack's mental state model of plans [Pollack, 1990] to the situation in which two agents jointly have a plan to perform some action requiring actions by both agents. Pollack s definition of the individual plan of an individual agent …