A prospective cohort study
is a research study
that follows over time groups of individuals who are similar in some respects (e.g., all are working adults) but differ on certain other characteristics (e.g., some smoke and others do not) and compares them for a particular outcome
(e.g., lung cancer). It should be emphasized that prospective studies begin with a sample whose members are free of the disease or disorder under study (e.g., free of lung cancer or free of major depression). Given the individual differences that exist in a sample (e.g, some people smoke, others do not), all the individuals in the sample are followed over time. The incidence rates for the disease under study are ascertained in key subgroups. For example, in a sample that was free of lung cancer at the outset of the study, after 20 years, one may anticipate that the nonsmokers have the lowest 20-year incidence rate of the disease, moderate smokers, the next highest rate, and the heavy smokers, the highest rate. The prospective study is important for research on the etiology of diseases and disorders in humans because for ethical reasons people cannot be deliberately exposed to suspected risk factors in controlled experiments.
It can be more expensive than a case–control study.