Relative risk and odds ratio

In a cohort study, the relative risk of a disease can be calculated because all patients under study are available for the analysis comparing risk in exposed and unexposed individuals. In contrast, in a case-control study, cases are drawn from a population of patients who have the disease and controls from a population without the disease. Thus, the relative risk cannot be calculated, since the samples do not necessarily reflect the true proportion of patients who have been exposed to a risk factor. Instead, the relative risk can be estimated using the odds ratio.