Non-diseased subjects are enrolled and their baseline exposure status is ascertained and they are followed over time. Eventually, one can divided the cohort into groups based on an exposure and then compare their incidence of specific outcomes.



The characteristic feature of a cohort study is that the investigator identifies subjects at a point in time when they do not have the outcome of interest and compares the incidence of the outcome of interest among groups of exposed and unexposed (or less exposed) subjects. (We can refer to the groups being compared as exposure cohorts.) Cohorts may be identified retrospectively or prospectively, but in either case the outcome status needs to be established at least twice. It must be established that a cohort did not have the outcome of interest at the beginning of the observation period, and the cohort needs to be examined again to determine whether or not the outcome subsequently developed, i.e., the incidence in each of the exposure groups.

Learning Objectives

Upon successful completion of this section of the course, the student will be able to:

- Prospective cohort study

- Retrospective cohort study

- Ambidirectional study 

- An internal comparison group

- An external comparison group

- A general population comparison group