A cohort study is a type of longitudinal study in which researchers follow a group (cohort) of participants for a certain duration (several years to a decade or more). Often, the participants in a cohort study are recruited because they share similar occupational, demographic, or lifestyle characteristics (e.g., being smokers or shift workers, or having obesity). The researchers conducting a cohort study record behaviors and exposures that they think could influence the health outcomes that they’re studying and try to determine whether they’re correlated.

Although the associations that a cohort study can describe aren’t causal, they still provide a quantitative description of the risk of an outcome (like lung cancer) given an exposure (like smoking) In fact, cohort studies were integral in establishing a definitive link between smoking and lung cancer — although it can’t prove smoking causes lung cancer, the association between the two was so strong that it couldn’t be ignored. Some of the most well-known cohort studies include the National Health and Nutritional Examination Survey (NHANES) and the Nurses’ Health Survey (NHS).