Building and Testing the SHYSTER-MYCIN Hybrid Legal Expert System

Abstract

SHYSTER-MYCIN is a hybrid legal expert system created by combining rule-based and case-based reasoning. The MYCIN part uses a system of rules to reason with provisions of an Act of a parliament; the SHYSTER part uses analogy to reason with cases that explain "open-textured" concepts encountered in legislation.

The construction of the expert system is focused upon: creating and evaluating a model of legal reasoning, and improving the reporting made by the MYCIN part.

The model of legal reasoning is supported by jurisprudential discussion. The model holds that rules (in the strict sense of the word) cannot be extracted from cases. Cases should therefore be argued by analogy. The only rules that exist in law are those in legislation.

The method of evaluating the model of legal reasoning is comparative. Reports by the system are compared with reports by a test group of legally trained people. Both the system and the test group were provided with the same material on which to base their reports. This ensured that the evaluation was of the model of reasoning, rather than the depth of knowledge.

The reporting made by MYCIN was improved for use in SHYSTER-MYCIN, so that the system states how it comes to its conclusions. This reporting was then restricted to only the "interesting" conclusions.

Keywords: law, legal, expert systems, artificial intelligence, jurisprudence, legal reasoning, hybrid, evaluation, testing, MYCIN, SHYSTER