Cohort studies: following people forward in time

A cohort study starts from the exposure and follows people forward to see who develops the outcome. Because exposure is measured first, it establishes timing, and because the whole population at risk is counted, it measures true incidence. Prospective cohorts take years. Retrospective cohorts use existing records, so a decade of ART outcomes in KwaZulu-Natal could be analyzed in months, limited to whatever clinicians wrote down. The central threat is loss to follow-up that is related to both exposure and outcome. Exposures that change over time are another threat, and often overlooked.

Also covered: prospective and retrospective cohorts · incidence · loss to follow-up · time-varying exposures

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Lesson 3 · 6 steps

1What descriptive designs can and cannot tell us7:16 2Cross-sectional studies and the problem of timing4:51 3Case-control studies: starting from the outcome6:21 4Cohort studies: following people forward in time6:39 5Controlling confounding in observational data7:49 6Choosing and reading an observational design6:45

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