Perhaps the most common and familiar way that the results of medical research and epidemiologic investigations are summarized is in a table of counts. Numbers of subjects with and without the outcome of interest are listed for each treatment or risk factor group. By using the study sample data thus tabulated, investigators quantify the association between treatment or risk factor and outcome. Three simple statistical calculations are used for this purpose: difference in proportions, relative risk, and odds ratio. The appropriate use of these statistics to estimate the association between treatment or risk factor and outcome in the relevant population depends on the design of the research. Herein, the enumeration of proportions, odds ratios, and risks and the relationships between them are demonstrated, along with guidelines for use and interpretation of these statistics appropriate to the type of study that gives rise to the data. Copyright RSNA, 2004