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Cohort studies are observational (no intervention) and longitudinal: they start from the exposure and follow participants over time to assess the outcome. They are the studies of risk measures — and of survival as well.

30-second summary

The cohort starts from the exposure and follows to the outcome.
It is the design of risk and survival measures.
It needs large samples and long follow-up — unfeasible for rare diseases.

Exposedlow fiber intake

Non-exposedhigh fiber intake

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Years of follow-up⌛ time passes

→

Outcomewho developed colorectal cancer?

The cohort looks forward: from today’s exposure to tomorrow’s outcome.

✓ Advantage: excellent for building causality hypotheses — the exposure precedes the outcome.

✗ Disadvantages: requires a large sample and long follow-up; becomes unfeasible for rare diseases.

Mortality, complication rates and survival in surgery are typically estimated in cohorts.

Frequently asked questions

Can a cohort be retrospective?

Yes: if exposure data were recorded in the past, the cohort is historical (retrospective), keeping the exposure → outcome logic.

Which measures come from a cohort?

Relative risk, incidence and survival curves — direct measures of risk.

What is the cohort's main enemy?

Loss to follow-up: patients who disappear may differ from those who stay, distorting the result.

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Written by Francisco Tustumi, digestive system surgeon (FMUSP), associate editor of ABCD.

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ABCD – BRAZILIAN ARCHIVES OF DIGESTIVE SURGERY is a periodic with a single annual volume in continuous publication, official organ of the Brazilian College of Digestive Surgery - CBCD. Technical manager: Dr. Francisco Tustumi | CRM: 157311 | RQE: 77151 - Cirurgia do Aparelho Digestivo

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