The goal of this column is to help working clinicians understand the statistical calculations involved in interpreting the results of diagnostic questionnaires. Using the Patient Health Questionnaire-9 as an example, the author explains how to determine the most appropriate cutoffs to choose, depending on the population involved, the probabilities of error (alpha and beta), and the expected losses associated with each kind of error in the context in which the test is being administered. (Journal of Psychiatric Practice 2011;17:57-60)