What does a retrospective cohort study involve?

• A. Past → Present; existing records; cheaper; faster; outcomes may have already occurred
• B. Present → Future; begin data collection
• C. Randomly assign exposure
• D. Cross-sectional snapshot

Answer: (A)

In a retrospective cohort study, researchers start with past exposure information from existing records and then look at outcomes that have already occurred by the time the study is done. This approach is cheaper and faster because the data are already collected in sources like medical records or databases, so the exposure status and outcomes are determined from the past relative to the study start. This contrasts with following people from now into the future (prospective cohort), assigning exposures (randomized trials), or taking a single-point snapshot of exposure and outcome (cross-sectional).