  Attempts to replicate probabilistic reasoning in expert systems have
typically overlooked a critical ingredient of that process. Probabilistic
analysis typically requires extensive judgments regarding interdependencies
among hypotheses and data, and regarding the appropriateness of various
alternative models. The application of such models is often an iterative
process, in which the plausibility of the results confirms or disconfirms the
validity of assumptions made in building the model. In current expert systems,
by contrast, probabilistic information is encapsulated within modular rules
(involving, for example, "certainty factors"), and there is no mechanism for
reviewing the overall form of the probability argument or the validity of the
judgments entering into it.

Artificial Intelligence