What fundamental temporal relationship does a cohort study establish between exposure and outcome?

• A. It follows exposed and unexposed groups over time to see who develops the outcome, establishing temporality.
• B. It compares cases with controls at a single point in time.
• C. It randomizes participants to exposure groups.
• D. It uses existing records only.

Answer: (A)

Exposure preceding the outcome is shown by starting with groups defined by whether they were exposed and then following them over time to see who develops the outcome. This forward-looking, longitudinal follow-up lets us observe incidence in each group and confirm that the exposure occurred before the outcome, which is the essential temporal relationship a cohort study provides. Other designs don’t establish this timing as clearly: a single-point snapshot can’t confirm which came first, randomized assignment is an experimental approach rather than a cohort design, and relying on existing records can be retrospective without guaranteed clear timing between exposure and outcome.