Base Rate Analysis in Cross-Cultural Clinical Psychology: Diagnostic Accuracy in the Balance

Most clinicians would agree that an accurate diagnosis of psychological disorders depends upon sound clinical judgment, combined with the outcome of reliable and valid diagnostic tests. Another, often overlooked factor that has a direct influence on accuracy of diagnosis is the base rate of the disorder type in the population. Base rates are defined as the number of people with the condition divided by the number of people in the reference population, and are a statistic reflecting current population prevalence. In this chapter, we consider the influence of base rates on diagnostic accuracy, with special emphasis on the effect of differential base rates in minority and specialty groups on the diagnostic enterprise. In the first section, we discuss base rates from a historical and practical perspective. To illustrate the influence of base rates on diagnostic accuracy in special populations, we consider the differential diagnosis of Alzheimer’s disease (AD) and multi-infarct dementia (MID) in Caucasian and African-American populations. Next, we present information on differential diagnosis of schizophrenia among persons with and without mental retardation (MR). These examples are used to demonstrate how base rate information can be used to aid in the selection of diagnostic measures, and also to show the dramatic impact that differing base rates can have on overall diagnostic accuracy. The chapter concludes by revisiting the issues of differential base rates in cross-cultural psychology.