Cohort Study

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Cohort Study. Martin Barraud / Getty Images

Definition: A cohort study is a type of observational study that follows a group of people (a cohort, from the Latin for yard, or retinue) over time. Cohort studies can either be prospective or retrospective. In a prospective cohort study, the cohort is formed and then followed over time. In a retrospective cohort study, data is gathered for a cohort that was formed sometime in the past - usually in a group where good records have been kept and all exposures and outcomes have already occurred.

For example, a study that starts in 2015 to look at the sexual health of young people who started attending a particular primary school in 2000 would be a retrospective cohort. A study that starts in 2015 with a group of college graduates and follows them as they aged would be a prospective cohort study.

In both retrospective and prospective cohort studies, the goal is to break down the cohort into various groups of exposed and non-exposed individuals and follow their outcomes over time. In a large enough cohort study, many exposures and outcomes may be evaluated at once. In smaller cohort studies, there may only be one or two exposures, such as smoking or results on an early test.

Some famous examples of cohort studies include:

  1. The Nurses' Health Study - Initially focused on cancer prevention, the nurses health study has done research on many aspects of women's health.
  2. The Framingham Heart Study - A study of cardiovascular disease and its risk factors that has been going on for more than 60 years.
  1. The Physicians' Health Study - Started as a way to look at whether aspirin and beta-carotene could prevent cardiovascular disease and cancer.

Examples: The Nurses' Health Study is a good example of a prospective cohort study. In this study, groups of nurses have been followed for over 30 years to see how various factors - including smoking, hormone levels, and exercise - affect their long-term health.

The far more problematic Tuskeegee Syphilis Study, properly called the "Tuskegee Study of Untreated Syphilis in the Negro Male," was also a prospective cohort study. It followed a group of African American men with syphilis to see what happened to their health over time. Unfortunately, the ethically questionable study did not provide treatment to the men, even once it became available.


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Stampfer MJ, Willett WC, Colditz GA, Speizer FE, Hennekens CH. A prospective study of past use of oral contraceptive agents and risk of cardiovascular diseases. N Engl J Med. 1988 Nov 17; 319(20):1313-7.

Tuskeegee University. About the USPHS Syphilis Study. Accessed 9/9/15 at

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