1. In a method for collecting information by tracing a rule base of an expert system with a data processing system having a processing unit which functions with said expert system program as an inference engine, the improvement comprising the following steps in combination, (a) establishing in said rule base, a rule tree having a goal node and at least one evidence node which functions to present a class question to a user of said system, and, (b) selectively setting the answer in said one evidence node without obtaining said answer in response to asking said class question to said user, said step of setting the answer including the step of determining the answer to be set in accordance with information obtained from at least one other node in said rule base which does not ask the same class question.