The analytic method of epidemiologic study in which subsets of a defined population can be identified as exposed to a factor(s) hypothesized to influence the probability of occurrence of a given disease or other outcome (Last, p.33)

• Ambidirectional Cohort: A cohort study that shares components of both the prospective and retrospective designs (Aschengrau & Seage, pp. 206-208)
• Prospective Cohort (a.k.a., Concurrent Cohort Study): Participants are grouped on the basis of past or current exposure and are followed into the future in order to observe the outcome(s) of interest. (Aschengrau & Seage, pp. 206-208)
• Retrospective Cohort: Both the exposures and outcomes have already occurred by the time the study begins (Aschengrau & Seage, pp. 206-208)