Which study design allows an INTERVENTION to be introduced into the study?

Correct answer: D. Randomized clinical trial (RCT)

• A. Cross-sectional studies observe participants at one time point — no intervention is introduced.
• B. Cohort studies observe what happens naturally over time — while prospective cohorts can follow participants, they do not introduce a controlled intervention.
• C. Case-control studies retrospectively look at disease status and exposures — no intervention.
• D. Correct. An RCT is the only study design where an INTERVENTION (treatment, exposure, procedure) is introduced — participants are randomly assigned to intervention or control groups, and outcomes are measured. This is what makes RCTs the gold standard for establishing causation.