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Lead Author(s): Jeff Martin, MD

Definition of a Cohort

Any well defined population can be thought of as a cohort that continues to recruit new subjects during the time period of the study.

Prospective Cohort

In a prospective cohort, individuals are identified and entered into the cohort before the observation time occurs

Retrospective Cohort

In a retrospective cohort, the observation time and events have already occurred by the time that the investigator decides to do the study.

Closed Cohort

Many cohorts are closed. The investigators recruit a population at baseline and follow them for some period of time and no new subjects are enrolled.

Open Cohort

Other cohorts are open (also called dynamic) in the sense that new members may be recruited as the cohort follow-up progresses (this is always true to some degree since at the beginning of a cohort all the members are almost never recruited on the same day and recruiting may be a lengthy process in some instances).
  • For example, the San Francisco City Clinic Cohort Study of HIV/AIDS recruited additional men at several subsequent years after the baseline year; first, to increase the sample size and then later to increase the representation of minority ethnicities.
  • A population like the members of Kaiser Permanente during some specified time period can be viewed as an open cohort because there new members are constantly being added.

Hypothetical Cohort

By a hypothetical cohort we mean that a cohort study was not performed, but that some possible or hypothetical cohort of individuals among whom the disease diagnoses were made can be defined.
  • That group of individuals could in theory have been enrolled in a cohort study.
  • An example would be the members of the Kaiser Permanente HMO at some point in time.
  • If all the members of Northern California Kaiser Permanente, say, had been enrolled in a cohort study of a particular disease five years ago, measurement of the disease outcome and possible predictors of disease during the past five years would provide cohort study data.
Topic revision: r6 - 02 Jun 2009 - 16:16:50 - MaryB?

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