Relative excess risk: An alternative measure of comparative risk

The proverbial relative risk may not always be the most suitable measure to compare the risk of two exposures, since it inherently includes a backgrou nd effect. An alternative comparative measure, the relative excess risk, is introduced. It applies to situations in which an "unexposed" reference gro up is included in addition to the two exposures under evaluation. This comp arative measure is based solely on the component of risk due to the exposur es, since it removes the background risk. Estimators of the relative excess risk are presented, along with formulas for the confidence intervals under cohort and case-control designs, using both crude and adjusted rate ratios . This new measure is illustrated with data from epidemiologic studies of t he risks of oral contraceptives and antidiabetic drugs.