Which of the following is the best individual study design?

Correct answer: A. Randomized Controlled Trial (RCT)

• A. The RANDOMIZED CONTROLLED TRIAL (RCT) is the gold standard single study design. It uses random allocation to treatment or control groups, eliminating selection bias, and allows for causal inference. The RCT is the highest-quality individual study type — above it on the evidence pyramid is only the systematic review/meta-analysis that COMBINES multiple RCTs.
• B. A cohort study is observational — better than case-control and case reports, but below the RCT in the evidence hierarchy.
• C. A case study (case report) describes a single patient and is at the BOTTOM of the evidence hierarchy — it cannot establish causation.
• D. A narrative literature review is a subjective summary of the literature and is prone to selection bias — it is not ranked high in the evidence hierarchy.