Which statement accurately differentiates prospective and retrospective cohort studies?

• A. Prospective cohorts collect past data; retrospective cohorts collect data going forward.
• B. Prospective cohorts collect exposure data going forward from baseline; retrospective cohorts use existing records to assemble exposure/outcome data from past to present.
• C. Prospective cohorts always require randomization.
• D. Prospective cohorts do not require follow-up.

Answer: (B)

The key idea is how data are gathered over time in a cohort study. In a prospective cohort, you identify participants now and collect exposure information as time moves forward from baseline, then follow everyone to see who develops the outcomes. In a retrospective cohort, you use existing records to reconstruct both exposure and outcome data from the past up to the present, rather than collecting new data going forward. This means the statement that correctly differentiates them says exposure data are collected going forward from baseline in prospective cohorts, while retrospective cohorts assemble exposure and outcome data from past records.

Note that randomization isn’t a feature of cohort studies, so requiring randomization isn’t correct, and a cohort study still involves follow-up to observe outcomes rather than having no follow-up.