Level 3a corresponds to which type of study?

• A. Systematic reviews of randomized controlled trials (RCTs)
• B. Individual cohort studies and low-quality RCTs
• C. Case-controlled studies
• D. Systematic reviews of case-control studies

Answer: (D)

In evidence hierarchy, higher-level evidence comes from designs that minimize bias, with systematic reviews of randomized trials at the top. When randomized evidence isn’t available or feasible, observational data provide insight. Case-control studies are efficient for studying rare outcomes but carry biases related to selection and recall. When multiple case-control studies are brought together in a systematic review, the result is a synthesized body of evidence that is stronger than any single case-control study but still falls below systematic reviews of randomized trials. That’s why this option—systematic reviews of case-control studies—is placed at Level 3a. It represents a consolidated assessment of case-control evidence, offering more reliability than individual case-control studies, yet not as definitive as reviews of randomized trials. The other options describe study designs or reviews that are either individual studies or reviews of different designs, which do not meet the level described by Level 3a.