Determining the accuracy of a medical test is quite difficult because accuracy is an elusive thing to estimate. It is well known that the accuracy of a particular test will vary because of intra and inter study variation. Even though a test can be replicated under identical conditions, nevertheless, the accuracy will vary between repetitions. The literature on the accuracy of a particular test continues to grow, with each study giving some estimate of its accuracy, but, of course, the estimates vary because of different study conditions. Accuracy varies because the study population of patients vary, the test itself changes somewhat, and the readers of the test scores also vary. Because of the various sources of variation, it is important to conduct studies that summarize the accuracy of a particular medical test.