Cohort study

A cohort study follows a defined group of people over time and records who develops the outcome of interest. Exposure is measured first and the outcome later, so the time order is clear. Researchers enrol people who do not yet have the outcome, sort them by exposure, then compare event rates between the groups. The usual summary is a hazard ratio from a Cox model, reported alongside person-years of follow-up. Prospective cohorts collect data forward from enrolment; retrospective cohorts reconstruct it from records that already exist. The Framingham Heart Study enrolled 5,209 residents in 1948 and produced the modern idea of a cardiovascular risk factor. Doll and Hill followed roughly 34,000 British doctors from 1951 and tied smoking to lung cancer. Current cohorts are far larger: UK Biobank recruited about 500,000 adults aged 40 to 69 between 2006 and 2010. The main weakness is confounding. People who eat more vegetables also smoke less and move more, and no adjustment removes every difference. Reverse causation is the second trap, because early illness lowers body weight and makes low weight look dangerous. Loss to follow-up distorts results when it tracks exposure. In longevity and nutrition research cohorts carry most of the evidence, since no trial can randomise a diet for 30 years.

Last reviewed: August 25, 2026

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