Tests can be used either diagnostically (i.e., to confirm or rule out the presence of a condition in people suspected of having it) or as a screening instrument (determining who in a large group of people has the condition and often when those people are unaware of it or unwilling to admit to it). Tests that may be useful and accurate for diagnosis may actually do more harm than good when used as a screening instrument. The reason is that the proportion of false negatives may be high when the prevalence is high, and the proportion of false positives tends to be high when the prevalence of the condition is low (the usual situation with screening tests). My first aim of this article is to discuss the effects of the base rate, or prevalence, of a disorder on the accuracy of test results. My second aim is to review some of the many diagnostic efficiency statistics that can be derived from a 2 x 2 table, including the overall correct classification rate, kappa, phi, the odds ratio, positive and negative predictive power and some variants of them, and likelihood ratios. In the last part of this article, I review the recent Standards for Reporting of Diagnostic Accuracy guidelines (Bossuyt et al., 2003) for reporting the results of diagnostic tests and extend them to cover the types of tests used by psychologists.