A prospective cohort study follows a group or cohort of individuals who are initially free of the outcome of interest. Individuals in a cohort generally share some underlying characteristic, such as age, sex, or exposure to a risk factor. Some studies may comprise several different cohorts. The study is usually conducted for a predetermined period, long enough for some members of the cohort to develop the outcome of interest. Individuals who developed the outcome are compared with those who did not. The report of the study should include a description of the cohort and the length of follow-up, what independent variables were measured and how, and what outcomes were measured and how. The number of individuals unavailable for follow-up and whether they differed from those with complete follow-up should also be included. All adverse events should be reported.
Any previous published reports of closely related studies from the same cohort should be cited in the text or should be clear from the study name (eg, the Framingham Study). All previous reports on the same or similar outcomes should be cited.
Retrospective cohort studies may be appropriate if investigators are blinded to study outcomes when formulating the hypothesis and determining the dependent and independent variables, but many of the strengths of prospective cohort studies are lost with retrospective studies, such as identifying the population to study and defining the variables and outcomes before the events occur.