What is a cohort study?
A cohort is a group sharing a common characteristic. In a cohort study, researchers measure exposures—habits, lab values and so on—in people who are disease-free at the start, then follow them for years or decades to see who gets sick. Unlike a randomized controlled trial, researchers don't assign the exposure.
Prospective vs. retrospective
- Prospective cohort: recruits people now and follows them forward. Data quality is high, but it takes time and money.
- Retrospective cohort: uses existing records (health checkups, insurance data) to reconstruct a past period. Faster, but key information may be missing.
Landmark examples
The Framingham Heart Study, launched in 1948 in Framingham, Massachusetts, helped establish high blood pressure, cholesterol and smoking as heart disease risk factors. Richard Doll and Austin Bradford Hill's British Doctors Study is the classic cohort that linked smoking to lung cancer.
What should you watch for when interpreting cohort studies?
Cohort studies show associations, not proof of cause. People who exercise a lot also tend to smoke less, so other factors (confounders) can creep into results; researchers try to reduce this with statistical adjustment. Reverse causation—early symptoms changing behavior—and the healthy-user effect, where health-conscious people take supplements, can also distort findings. Results are usually reported as relative risks or hazard ratios, and several cohorts are sometimes combined in a meta-analysis.
FAQ
- Why not just run an RCT instead?
- Many questions—like the long-term effects of smoking or diet—would be too slow, costly or unethical to randomize, so cohort studies fill that gap.
Sources reviewed: 2026-10-25
References
- Dawber, T. R., Meadors, G. F., & Moore, F. E. (1951). Epidemiological approaches to heart disease: the Framingham Study. American Journal of Public Health, 41(3), 279–286.
- Doll, R., & Hill, A. B. (1954). The mortality of doctors in relation to their smoking habits. BMJ, 1(4877), 1451–1455.
- Rothman, K. J. (2012). Epidemiology: An Introduction (2nd ed.). Oxford University Press.