Evidence-appraisal glossary
Cohort study
A cohort study follows a group of people over time, comparing those with and without an exposure to see who later develops an outcome. Because exposure is measured before the outcome appears, cohort studies can establish that the exposure came first, though they do not randomly assign it.
Also called: longitudinal study, prospective cohort, follow-up study.
In a cohort study, researchers define groups by their exposure status (for example, smokers versus non-smokers) and track them forward to record who develops the outcome of interest. Prospective cohorts enroll people and follow them into the future; retrospective cohorts reconstruct exposure and outcome from existing records. Because exposure is recorded before the outcome, the design supports statements about temporal order, and it can measure incidence and relative risk directly. When reading a cohort study, check how completely people were followed (large loss to follow-up can distort results), whether exposure and outcome were measured the same way in both groups, and which confounders were adjusted for. A classic example is the British Doctors Study, which followed physicians for decades and linked smoking to lung cancer by comparing outcome rates across exposure groups. The key question the design lets you ask: did the exposed and unexposed groups differ in the outcome after accounting for other differences between them?
This is a plain-language methodology definition for reading research. It is general education, not medical advice.