Cohort Studies

In Romans' time, a cohort of legionnaires consisted of a group of soldiers sharing the same military events for a certain period of time. In epidemiology we consider that a cohort consists of people belonging to the same population and sharing similar experience for a defined period of time.

Cohort studies involve the comparison of disease incidence over time (risk or rate) between two subsets of a population (two cohorts). One of the 2 cohorts is exposed to a certain characteristic (exposure). The other is not. All other things being equal between the two cohorts but for their exposure. In both cohorts we measure occurrence of disease over the specific study period. However whenever the condition of "all other things being equal" is not met, the comparison might be wrong.

The following graph adapted from Rodrigues [1] illustrates occurrence of cases over time in the two cohorts. Initially Ne persons are exposed and Nu persons are unexposed. The number of persons who are disease free decreases over time (shaded area). The number of cases (non shaded area) increases over time but more in the exposed cohort. At the end of the study, respectively Ce and Cu cases have occurred in the exposed and unexposed cohorts. The shaded area represents the cumulative time during which persons were at risk of developing disease in each of the cohorts during the entire follow up period.



[1] Rodrigues L. Kirkwood BR. Case-control designs in the study of common diseases: updates on the demise of the rare disease assumption and the choice of sampling scheme for controls. Int J Epidemiol 1990 Mar:19(1):205-13