Date of Award


Degree Type


Degree Name

Doctor of Philosophy in Psychology



First Advisor

W. Grant Willis


This dissertation generalized the base-rate fallacy in diagnostic decision-making to the field of school psychology. This fallacy involves allowing information other than prior probabilities inappropriately to dominate probability judgments. Participants included 80 school psychologists who were asked to rate the probability of a learning-disability diagnosis and to state their degree of confidence in that diagnosis. In Study I, participants received only base-rate information. In Study 11, participants received either (a) relevant base-rate information plus irrelevant individuating information, (b) relevant base-rate information plus relevant diagnostic information, or (c) relevant base-rate information plus relevant diagnostic information plus irrelevant individuating information. Additionally, in Study II these three levels of information were completely crossed with a second independent variable, that is, salient link. Here, base-rate information was either linked or not linked with the diagnosis to be predicted. Results showed that (a) school psychologists appropriately used relevant base-rate information only when no other information was available, (b) when a salient link between relevant base-rate information and the diagnosis to be predicted was provided, diagnostic accuracy did not change, (c) when relevant diagnostic and irrelevant individuating information were provided in addition to base-rate information, diagnostic accuracy decreased relative to base-rate-only information, and (d) school psychologists were least confident of diagnoses for which they demonstrated most accuracy. Results were discussed in terms of implications for diagnostic decision making in the field of school psychology.



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