The proper interpretation of research findings in obstetrics and gynecology increasingly requires some understanding of epidemiology and biostatistics. The disciplines of epidemiology and biostatistics are inextricably related; the goal of epidemiology is accurate measurement of the relationship between an exposure and a disease, and statistical methods are required for achieving that objective. Most epidemiologic studies in the obstetrics and gynecology literature can be classified as 1) cross-sectional, 2) case-control, or 3) cohort (follow-up) studies. The 2 x 2 table represents the basic analytic format for all three types of epidemiologic studies. Information from this table can be used to estimate both the magnitude of the exposure-disease relationship and the relative likelihood that chance explains study findings. Accurate measurement of the relationship between an exposure and a disease can be impeded by two major sources of error: bias and chance. In broad terms, biases can be classified as those related to 1) selection, 2) information, and 3) the presence of extraneous variables. Because biases in epidemiologic studies distort measurements, they must be identified, characterized, and, if possible, avoided. When biases cannot be avoided, knowledge of their likely impact on study findings must be assessed. The role of chance is evaluated by statistical testing of the null hypothesis, ie, the hypothesis that two factors are not associated.(ABSTRACT TRUNCATED AT 250 WORDS)