Relative risk and odds ratio

In prospective cohort studies, the RR of a disease can be calculated because all patients under study are available, and it is therefore possible to calculate disease risk in exposed and unexposed individuals. By contrast, in case-control studies, cases are drawn from a population of patients who have the disease and controls from a population without the disease. Thus, the RR 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 RR can be approximated by calculating the OR as shown above. This approximation is most accurate when the disease is rare. For common diseases, the OR is a poor estimate of RR.