Report Number: CS-TR-81-896
Institution: Stanford University, Department of Computer Science
Title: The epistemology of a rule-based expert system: a framework
Author: Clancey, William J.
Date: November 1981
Abstract: Production rules are a popular representation for encoding
heuristic knowledge in programs for scientific and medical
problem solving. However, experience with one of these
programs, MYCIN, indicates that the representation has
serious limitations: people other than the original rule
authors find it difficult to modify the rule set, and the
rules are unsuitable for use in other settings, such as for
application to teaching. These paroblems are rooted in
fundamental limitations in MYCIN's original rule
representation: the view that expert knowledge can be encoded
as a uniform, weakly-structured set of if/then associations
is found to be wanting.
To illustrate these problems, this paper examines MYCIN's
rujles from the perspective of a teacher trying to justify
them and to convey a problem-solving approach. We discover
that individual rules play different roles, have different
kinds of justifications, and are constructed using different
rationales for the ordering and choice of premise clauses.
This design knowledge, consisting of structural and strategic
concepts which lie outside the representation, is shown to be
procedurally embedded in the rules. Moreover, because the
data/hypothesis associations are themselves a proceduralized
form of underlying disease models, they can only be supported
by appealing to this deeper level of knowledge. Making
explicit this structural, strategic and support knowledge
enhances the ability to understand and modify the system.