Diagnostic testing is an important component of modern medical care. Unfortunately, many diagnostic tests are not rigorously evaluated before general application. Studies examining test characteristics often have methodologic flaws that impair their ability to provide reliable information on test performance. These flaws can introduce systematic nonrandom errors (biases) that distort measures of test accuracy. Other design errors can make it difficult to generalize the results of individual studies. These problems may enhance the apparent performance of poor tests while obscuring the performance of good tests, and they may result in the widespread use of tests with uncertain or limited efficacy. This article explores the ways in which studies of diagnostic test efficacy can be affected by bias and variability.