A retrospective cohort study is characterized by which timeline?

• A. Present → Future
• B. Past → Present; existing records; cheaper; faster; outcomes may have already occurred
• C. Present → Past
• D. Future → Past

Answer: (B)

Retrospective cohort studies use historical data to determine exposure status in the past and then look at outcomes that have already occurred up to the present. The timeline runs from Past to Present, which is why this option is the best fit. Using existing records makes the study cheaper and faster, since you’re not collecting new data or waiting for events to happen. A trade-off is that some outcomes may have already occurred by the time the study starts, and the data quality can be limited by what was recorded. In contrast, following people from present into the future describes a prospective cohort, and timelines like Present → Past or Future → Past don’t align with how this design is structured.