An Exploration of the Base Rate Scores of the Millon Clinical Multiaxial Inventory-III

The Millon Clinical Multiaxial Inventory (3rd ed.; MCMI-III) is a widely used psychological assessment of clinical and personality disorders. Unlike typical tests, the MCMI-III uses a base-rate score transformation to incorporate prior probabilities of disorder (i.e., base rates) in test output and diagnostic thresholds. The authors describe the base rate transformation and contend that its supporting documentation in the MCMI-III manual is incomplete and fails to meet interdisciplinary test documentation standards. They show that the MCMI-III's base rate transformation is not optimal, and they derive an optimal alternative transformation using Bayes' theorem. Bayes transformation formulae for 7 exponential family distributions are given. The authors discuss the effect of the base rate transformation and further argue that the MCMI-III's use of a single base rate per diagnostic category is ill-advised. They argue that base rates differ among clinical settings and that tests like the MCMI-HI should flexibly incorporate a base rate of disorder pertinent to the examinee's characteristics, such as demographics, chief complaint, clinical history, or other variables. They explain how this can readily be accomplished.