An expert system shell is an expert system without

Concept

An expert system separates into two independent halves. One half is the knowledge base: the facts, production rules and heuristics elicited from human experts about one particular subject area. The other half is the general problem-solving machinery, namely the inference engine, the user interface, the knowledge-acquisition module and the explanation facility, all of which are written in terms of the rule format rather than in terms of any one subject.

An expert system shell is that second half packaged and delivered on its own: a reusable, empty framework into which a knowledge base can later be loaded.

Application

Since the shell is shipped before any rules exist for it, the component it does not carry is the subject-specific knowledge. To build a new expert system a developer elicits rules for the chosen field, such as medicine, geology or credit assessment, and loads them into the same shell; the chaining, questioning and explanation machinery is reused unchanged across all of those fields. An expert system shell is therefore an expert system without domain knowledge.

Cross-check

Test that claim against each of the other components named among the choices:

• explanation facility: the shell supplies it, because tracing which rules fired to reach a conclusion is defined over the rule format itself and needs no subject-specific content.

• reasoning with knowledge: the shell supplies it, because forward chaining and backward chaining are procedures over rules, not over any particular field's facts.

• all three absent together: a package carrying neither an inference engine nor an explanation module would leave nothing reusable to ship, which contradicts the purpose of a shell.

A concrete illustration is EMYCIN, short for Essential MYCIN, which was produced by stripping the medical knowledge base out of MYCIN and keeping the remaining machinery; it became one of the earliest widely used shells.

Result

An expert system shell equals an expert system minus its domain knowledge.