The Base-Rate Fallacy in Probability Judgments

Abstract
The base-rate fallacy is people's tendency to ignore base rates in favor of case-specific information (when such is available), rather than integrate the two. This tendency has important implications for understanding judgment phenomena in many clinical, legal, and social-psychological settings. According to the account suggested in this paper, people order information by its perceived degree of relevance, and let high-relevance items dominate low- relevance items. Information that relates more specifically to the judged target case or is causally linked to it is deemed more relevant than general background data, thus yielding the base--rate fallacy in typical Bayesian inference problems. A large series of probabilistic inference problems was presented to subjects, in which relevance was manipulated in various ways, and the empirical results confirm the above account. In particular, base rates will be combined with other information when the two kinds of information are made to appear equally relevant.

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