Modern diagnostic technologies such as cancer screening tests or forensic DNA tests confront users with a basic problem, namely that of inferring the accuracy of a diagnostic test on the basis of statistical information about the test. The required statistics can be presented in a wide variety of ways. While these modes of presentation of statistical information are mathematically equivalent, psychologically they are not. This chapter shows that supplementing diagnostic technology with psychological knowledge about how people process frequency information provides us with a very simple — but powerful — method for improving diagnostic inferences.