Epidemiology

Measures of disease association include relative and attributable risk. Relative risk estimates the magnitude of an association between exposure and disease, based on the incidence of disease in the exposed group relative to the unexposed group. A relative risk of 1.0 indicates that there is no association between the exposure and outcome; a relative risk of greater than 1.0 indicates a positive association or increased risk; and a relative risk of less than 1.0 indicates an inverse association, or decreased risk (a protective effect). Incidence usually cannot be calculated in a case-control study, because participants are selected on the basis of disease. In this instance, the odds ratio (the ratio of odds of exposure among the cases to that of the controls) approximates the relative risk. Attributable risk, or risk difference, is the absolute difference in incidence between an exposed and unexposed group. It quantifies the risk of disease in the exposed group attributable to the exposure by removing the risk that would have occurred due to other causes. Expressed differently, attributable risk calculates the number of cases of disease among the exposed that could be eliminated if the exposure were eliminated. This is a useful measure of the public health impact of an exposure, assuming there is a cause-effect relationship. It is not possible to calculate attributable risk for most case-control studies, because incidence cannot be determined.