<Table> <Tr> <Td> </Td> <Td> This article includes a list of references, related reading or external links, but its sources remain unclear because it lacks inline citations . Please help to improve this article by introducing more precise citations . (June 2015) (Learn how and when to remove this template message) </Td> </Tr> </Table> <Tr> <Td> </Td> <Td> This article includes a list of references, related reading or external links, but its sources remain unclear because it lacks inline citations . Please help to improve this article by introducing more precise citations . (June 2015) (Learn how and when to remove this template message) </Td> </Tr> <P> The rare disease assumption is a mathematical assumption in epidemiologic case - control studies where the hypothesis tests the association between an exposure and a disease . It is assumed that, if the prevalence of the disease is low, then the odds ratio approaches the relative risk . </P> <P> Case control studies are relatively inexpensive and less time consuming than cohort studies . Since case control studies don't track patients over time, they can't establish relative risk . The case control study can, however, calculate the exposure - odds ratio, which, mathematically, is supposed to approach the relative risk as prevalence falls . </P>

When does the odds ratio approximate the relative risk