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What is a Cohort Study?

Cohort studies are observational studies that compare outcomes (disease/illness) between groups (exposed and unexposed) over time. These types of studies are efficient for rare exposures and can either be retrospective (looking backward in time) or prospective (looking forward in time). In retrospective cohort studies, some participants may already have the outcome of interest, whereas, in prospective cohort studies, the researcher will follow the group until the outcome(s) of interest develop.

What’s the difference between a retrospective cohort study and a case-control study?

In cohort studies, you already know what the exposure(s) are. Your objective is to determine if those exposures led to the outcome of interest (disease, death or some other outcome). In a retrospective cohort study, the exposed group may already have the outcome of interest, but you would examine them via historical medical records to observe them at a time before the outcome occurred. The goal here is to determine the risk of developing the outcome, given that they were exposed to the variable of interest.

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In a case-control study, on the other hand, our primary concern is to pinpoint the exposure that caused the outcome of interest. In case-control studies, you already know the outcome, but you do not know the exposure. A case-control study is the most efficient study design when the outcome is rare.

When is it useful to conduct a cohort study?

Cohort studies are useful for studying rare exposures. However, conducting a cohort study might not be the best approach when there is a long time to the development of a disease. Cohort studies give us the real incidence rates and relative risks.

Example of a cohort study

The Framingham Heart Study (FHS) [https://www.framinghamheartstudy.org/] is an example of a cohort study. The goal of FHS is to examine the factors that lead to cardiovascular disease (CVD). The study enrolled people who had not developed CVD and followed them and their children over time.

Final thoughts

In cohort studies, information can be collected on multiple exposures, and various outcomes of interest can be analyzed with minimal increase in costs.


The most significant threat to cohort studies is the loss to follow-up. Cohort studies can also be very time consuming, and expensive, especially if the researcher has to wait several years before diseases start to manifest in enough individuals to obtain accurate results.

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Also published on Medium.