Can model-based and case-based expert systems operate together?

In discussing diagnostic expert systems, there is an ongoing debate as to whether model-based systems are superior to case-based systems, or vice versa. Our experience has shown that there is no real need for debate because the two are not mutually exclusive and, to the contrary, complement each other. Current expert system technology is capable of two reasoning mechanisms, in addition to other mechanisms, into one integrated system. Depending on the knowledge available, and time and cost considerations, expert systems allow the user to decide the relative proportion of case-based to modal-based reasoning he/she wishes to employ in any given situation. Diagnostic support software should be evaluated by two critical factor groups, Ben-Bassat, et al., 1992: (a) cost and time to deployment; and (b) accuracy, completeness and efficiency of the diagnostic process. The question, therefore is: for a given budget of time and money, which approach will result in the best diagnostic performance-case-based, model-based, or combination of the two. In this paper we will discuss the role of expert systems in combining model based and case-based reasoning to effect the most efficient user defined solution to diagnostic performance.