Evidence guide
What Is a Cohort Study?
A cohort study follows a defined group to see how exposures relate to later outcomes.
Prospective and retrospective
Prospective cohorts measure exposures before outcomes occur; retrospective cohorts use existing records. Both are observational unless assignment is randomized.
What cohorts do well
They estimate incidence, study multiple outcomes and examine exposures that cannot be assigned ethically.
What weakens inference
Confounding, changing exposure, loss to follow-up, misclassification and reverse causation can all distort associations.
A practical reading checklist
- Was exposure measured before the outcome?
- How complete was follow-up?
- Were key confounders measured well?
- Were absolute risks reported?