<Tr> <Td> </Td> <Td> This section may require cleanup to meet Wikipedia's quality standards . No cleanup reason has been specified . Please help improve this section if you can . (July 2011) (Learn how and when to remove this template message) </Td> </Tr> <P> An example of an epidemiological question that can be answered using a cohort study is whether exposure to X (say, smoking) associates with outcome Y (say, lung cancer). In 1951 commenced the British Doctors Study, a cohort that included both smokers (the exposed group) and non-smokers (the unexposed group). The study continued through 2001 . By 1956, the study provided convincing proof of the association of smoking with the incidence of lung cancer . In a cohort study, the groups are matched in terms of many other variables such as economic status and other health status so that the variable being assessed, the independent variable (in this case, smoking) can be isolated as the cause of the dependent variable (in this case, lung cancer). In this example, a statistically significant increase in the incidence of lung cancer in the smoking group as compared to the non-smoking group is evidence in favor of the hypothesis . However, rare outcomes, such as lung cancer, are generally not studied with the use of a cohort study, but are rather studied with the use of a case - control study . </P> <P> Shorter term studies are commonly used in medical research as a form of clinical trial, or means to test a particular hypothesis of clinical importance . Such studies typically follow two groups of patients for a period of time and compare an endpoint or outcome measure between the two groups . </P> <P> Randomized controlled trials, or RCTs are a superior methodology in the hierarchy of evidence, because they limit the potential for bias by randomly assigning one patient pool to an intervention and another patient pool to non-intervention (or placebo). This minimizes the chance that the incidence of confounding variables will differ between the two groups . </P>

What type of research study if any is most strongly influenced by cohort effects