Level 2a is associated with which type of study?

• A. Systematic reviews of randomized controlled trials (RCTs)
• B. Individual RCTs with narrow confidence interval
• C. Systematic reviews of cohort studies
• D. Case-controlled studies

Answer: (C)

In this level of evidence framework, Level 2a corresponds to systematic reviews that synthesize cohort studies. A cohort study follows groups over time to observe how exposures relate to outcomes, and pooling multiple such studies in a systematic review increases precision and shows consistency across different populations. This strengthens the overall conclusion compared with any single cohort study, but it’s still considered weaker than a systematic review of randomized controlled trials, since randomization in RCTs more effectively controls bias and confounding. An individual RCT with a narrow confidence interval is classified as Level 1b, reflecting the strength of a single well-conducted trial. Case-controlled studies are a different observational design and generally fall lower in the hierarchy. Therefore, Level 2a is best matched by systematic reviews of cohort studies.