The weighing of pathological and non-pathological information in clinical judgment.

On the basis of the classic data of Meehl (1959), I examine how clinical psychologists use the MMPI scales to judge the degree of pathology of psychiatric patients by comparing linear models of the judgment to a linear model of the criterion (the actual diagnosis of the patients). This comparison reveals that excessively heavy weight is assigned to pathological information in comparison to non-pathological information. Additional analyses reveal that this biased weighing also influences the actual diagnosis and that it is a major determinant of the accuracy of clinical judgment. It is suggested that these effects arise from a confirmation bias associated with the hypothesis that a patient has severe, rather than mild, pathology.