Proceedings of the Annual Meeting of the Cognitive Science Society

We analyze a tradeoff between early warnings of plan failures and false positives. In general, a decision rule that provides earlier warnings will also produce more false positives. Slack time envelopes are decision rules that warn of plan failures in our Phoenix system. Until now, they have been constructed according to ad hoc criteria. In this paper w e show that good performance under different criteria can be achieved by slack time envelopes throughout the course of a plan, even though envelopes are very simple decision rules. W e also develop a probabilistic model of plan progress, from which w e derive an algorithm for constructing slack time envek>pes that achieve desired tradeoffs between early warnings and false positives.