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The Base-Rate Fallacy in School Psychology

Brown University School of Medicine

W. Grant Willis

University of Rhode Island

David Faust

University of Rhode Island

The diagnostic utility of a sign is dependent on the base rate, or prevalence, of an event within a population and may shift dramatically when the base rate changes. Decision makers, however, often commit the base-rate fallacy, that is, they underweight or disregard base-rate information and defer instead to case-specific information, even when it is irrelevant to the decision. The studies reported here examined how school psychologists integrated relevant and irrelevant clinical information with base rates, how diagnostic accuracy was influenced by enhancing the salience of base rates, and how base rates were used in the absence of other clinical information. Consistent with prior literature on the base-rate fallacy, although school psychologists used base-rate information appropriately when no other clinical information was available, their diagnostic accuracy declined markedly with the addition of either irrelevant or relevant clinical information. This effect was robust to and not reversed by the salience of base-rate information. This research also provides a demonstration of the important but counterintuitive notion that the addition of further valid information to the diagnostic mix can lower accuracy by decreasing attention to a weight assigned to more predictive variables.

Journal of Psychoeducational Assessment, Vol. 15, No. 4, 292-307 (1997)
DOI: 10.1177/073428299701500401


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