Cohort study

A cohort study is an observational research design in which investigators identify a group of people, the cohort, sort them by whether they have been exposed to some factor, and then follow them over time to see who develops a disease or other outcome. Nobody has the outcome at the start, and the researchers do not assign the exposure; they only record it. Comparing how often the outcome occurs in the exposed and unexposed groups shows whether the two are linked.

In a prospective cohort, people are enrolled now and followed into the future, sometimes for decades. In a retrospective cohort, researchers use existing records in which both the exposures and the outcomes have already happened. Because exposure is measured before the outcome, cohort studies show the order of events clearly, can track several outcomes at once and work well for rare exposures. However, they are slow and costly, lose participants along the way and are inefficient for rare diseases.

The Framingham Heart Study and the Nurses' Health Study are well-known examples that revealed major risk factors for heart disease and other chronic illnesses.