The World Wide Web facilitates user access to knowledge-based
decision support systems. Such web-enabled systems can provide users with
advice about how decision-makers exercise discretion. GetAid, developed using
the web-based shell environment WebShell, is an example of a web-based
decision support system operating in a discretionary legal domain. This paper
presents the Context, Criteria, Contingency evaluation framework for knowledge-
based systems, general in design but geared towards the evaluation of legal
knowledge-based systems. Central to this framework is a hierarchical model of
evaluation criteria arranged in four quadrants: verification and validation, user
credibility, technical infrastructure and the impact of the system upon its
environment. This framework frames an evaluation both in terms of the context
of use of the system and the context of its evaluation and includes guidelines for
the selection of appropriate evaluation criteria under differing contingencies. A
case study is presented describing the use of this evaluation framework in
planning the evaluation of the web-deployed GetAid system.