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Cohort Study

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Updated January 26, 2014

Definition: A cohort study is a type of observational study that follows a group of people 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.

In both retrospective and prospective cohort studies, the goal is to break down the cohort into a group of exposed and a group of non-exposed individuals and follow their outcomes over time. In a large enough cohort study, many exposures and outcomes may be evaluated at once.

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.
  3. 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.

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