Researchers in the development of medical expert systems have Increasingly recognized the Importance of explanation capabilities in encouraging the acceptance of their programs. One survey of potential users of medical advice systems has suggested that explanation may be the single most important capability of an acceptable clinical decision tool (16). Good explanations serve four functions in a consultation system: 111 they provide a method for examining the program's reasoning if errors arise when the system is being built; 121 they assure users that the reasoning is logical, thereby increasing user acceptance of the system; 131 they may persuade users that unexpected advice is appropriate; and 141 they can educate users in areas where their knowledge may be weak.

The ONCOCIN Project is supported by research grants from the National Library of Medicine, the Division of Research Resources of the NIH, the Office of Naval Re"arch, and the Henry J. Kaiser Family Foundation.

Symbolic inference since the time of Aristotle has involved the combination of symbolic expressions.

Multiple processors can be used to achieve a speedup of a backward-chaining deduction by distributing or-parallel deductions. However, the actual speedup obtained is strongly dependent on the task allocation strategy. Also, communication cost can be a significant part of the overall cost of a deduction. For the multiple processor scenario used in this paper,, processors with replicated databases on a broadcast network, a variable supply model (VSM) is presented. VSM represents an infinite class of strategies with varying communication requirements.

A broad range of heuristic programs--embracing forms of diagnosis, catalog selection, and skeletal planning--accomplish a kind of well-structured problem solving called classification. These programs have a characteristic inference structure that systematically relates data to a preenumerated set of solutions by abstraction.

Current subprojects include applications of the program Gerring.

MARS is an experimental system that provides a framework for implementing hierarchical discrete event driven simulators. The program is independent of any domain.

The duraticn of the neuro'ogical signs is 7.2 hours.

To perform these diagnoses, DART must frequently determine how the hardware's primary inputs can be manipulated to produce desired test conditions at internal nodes. Especially when the system's behavior is time-dependent, this reasoning must be carefully controlled, or a combinatorial explosion may result. This paper contrasts two techniques for representing time-dependent digital system behavior and controlling reasoning to achieve desired hardware states. 2

Table 1: Measured comparison of three control strategies.