The increased use of screening tests for drug use or antibodies to the HTLV-III (AIDS) virus, as well as pre-employment polygraph testing, has raised concerns about the reliability of the results of these procedures. This paper reviews the mathematical model underlying the analysis of data from screening tests. In addition to the known formulas for the proportion of positive (negative) classifications that are correct, we provide a large sample approximation to their standard errors. The results reinforce the need for confirmatory tests and indicate that moderately large sample sizes should be used to determine the accuracy rates of screening tests that will be applied to the general population in which the prevalence of the disease or trait is low.