Cohort studies are used to study how common diseases are, their causes, and their prognoses. Cohorts are groups of people who are selected on the basis of certain characteristics. For example, if exposure to a risk factor such as cigarette smoke is suspected to cause a disease, a cohort can be selected in which one group has been exposed and another group has not. Both groups are then studied for signs or symptoms of disease.
Cohort studies can be prospective (cohorts are identified before any signs of disease and are followed up over time) or retrospective (data is used that has already been collected, possibly over a long period of time).
Cohort studies are useful when it would be unethical to carry out a randomised controlled trial (RCT). For example, deliberately exposing people to cigarette smoke or asbestos would be unethical and therefore cannot be done.