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UBC Theses and Dissertations

Explanations in hybrid expert systems Scott, Lawrence Gill


This thesis addresses the problem of providing explanations for expert systems implemented in a shell that supports a hybrid knowledge representation architecture. Hybrid representations combine rules and frames and are the predominant architecture in intermediate and high-end commercial expert system shells. The main point of the thesis is that frames can be endowed with explanation capabilities on a par with rules. The point is illustrated by a partial specification for an expert system shell and sample explanations which could be generated by an expert system coded to that specification. As background information, the thesis introduces expert systems and the standard knowledge representation schemes that support them: rule-only schemes, and hybrid schemes that combine rules with frames. Explanations for expert systems are introduced in the context of rules, since rules are the only representation for which explanations are supported, either in commercial tools or in the preponderance of research. The problem addressed by the thesis, how to produce explanations for hybrid architectures, is analyzed in two dimensions. Research was surveyed in three areas for guiding principles toward solving the problem: frame logic, metalevel architectures, and reflective architectures. With the few principles that were discovered in hand, the problem is then analyzed into a small number of subproblems, mainly concerning high-level architectural decisions. The solution proposed to the problem is described in two ways. First a partial specification for expert system shell functionality is offered, which describes, first, object structures and, then, behaviors at three points in time—object compilation time, execution time, and explanation generation time. The second component of the description is a set of extended examples which illustrate explanation generation in a hypothetical expert system. The solution adopts principles of reflective architectures, storing metainformation for explanations in metaobjects which are distinct from the object-level objects they explain. The most novel contribution of the solution is a scheme for relating all the ways that objects' slot values may be computed to the goal tree construct introduced by the seminal Mycin expert system. The final chapter explores potential problems with the solution and the possibility of producing better explanations for hybrid expert system shell architectures.

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