It has been found that expert physicians use forward reasoning in diagnostic explanations of clinical cases. This paper shows that the Kintsch-Greeno model for solving arithmetic word problems, which assumes a forward chaining process, can be extended to explain this phenomena. The basic approach is to modify the lexicon and the schema structure of the existing simulation program while retaining the basic control structure. The principle modifications are in the structure of the schemata which make use of three slots: indicator,abnormality and consequence. As with the Kintsch-Greeno theory,the model proceeds by using these schemata to build super-schemata from the propositional representation of the problem text.

This paper examines the performance of cardiologists, psychiatrists, and surgeons indiagnostic explanations of cases within and outside their domain. The protocols were analyzed by techniques of transforming a propositional representation into a semantic network. Some graph theoretic criteria for analyzing semantic networks are used for precision of analysis. The results show that the subjects interpret cases in terms of the familiar component of the problem, using specific domain knowledge. This is related to forward directed reasoning. Unfamiliar or uncertaincomponents of the disorder are either ignored or explained using backward reasoning strategies. Atendency to move from a forward driven strategy to a backward driven strategy and vice versa is alsoseen in sc.ne protocols. This sequence is repeated a number of times to form a chain consisting offorward/backward reasoning sequences. This has implications for how subsequent patientinformation is processed in order to make decisions for treatment and management