Which statement correctly distinguishes prospective from retrospective cohort designs?

• A. Prospective cohorts collect historical records after outcomes occur.
• B. Retrospective cohorts collect data going forward in time.
• C. Prospective cohorts collect exposure data before outcomes occur; retrospective cohorts use existing records to reconstruct timelines.
• D. Both designs are always conducted with randomization.

Answer: (C)

The distinction being tested is how the timing and source of exposure information relate to outcomes in cohort studies. In a prospective cohort, you determine exposure status before outcomes occur and then follow people forward in time to see who develops the outcomes. This preserves the temporal order and often allows more accurate measurement of exposure and incidence. In a retrospective cohort, you use existing records to reconstruct the exposure and outcome timelines, essentially looking back in time from the present. This can be faster and cheaper but depends on the quality and completeness of past data.

The statement that best fits is that prospective cohorts collect exposure data before outcomes occur, while retrospective cohorts use existing records to reconstruct timelines. The other ideas misstate the design: prospective cohorts don’t rely on historical records after outcomes, retrospective studies don’t collect data going forward in time, and cohort designs are observational and do not involve randomization.