The main goal of many medical studies is to evaluate the effect of a treatment or the risk of disease under given conditions. This entry introduces and discusses measures of effect and/or risk when the factors or variables of interest are categorical (i.e., nominal) in nature. The discussion focuses on the case of dichotomous variables (factors), that is, those that take only two values that often indicate the presence or absence of a characteristic (disease, treatment, exposure, etc.). The data consist of information on n individuals who have been categorized according to the presence or absence of two factors. The information is presented in a 2 × 2 contingency table like the one in Table 1.

The numbers in the table denote the frequency of each ...