Which study design is MOST effective (for establishing causation)?

Correct answer: C. Randomized Controlled Trial (RCT)

• A. A case report describes a single patient and provides the lowest level of evidence — useful for hypothesis generation only.
• B. Case-control studies are retrospective and observational, more prone to bias (e.g., recall bias) than RCTs.
• C. Correct. RCTs randomize subjects to intervention/control groups, minimizing bias and confounding, making them the gold standard ('most effective') study design for establishing causality.
• D. Cohort studies are observational and prospective but still subject to confounding since exposure is not randomly assigned.