Level 2b corresponds to which study type?

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

Answer: (C)

Level 2b falls in the middle tier of study designs and represents evidence from studies that provide direct data from individuals but with more limitations than the strongest designs. Specifically, Level 2b includes individual cohort studies and low-quality randomized controlled trials. Cohort studies follow groups over time to observe outcomes after exposure, offering useful estimates but susceptible to bias and confounding. Low-quality RCTs may have problems like inadequate blinding, poor allocation concealment, small samples, or protocol deviations, which can bias results.

Because of these limitations, this level offers moderate evidence strength: informative and directly applicable, but not as robust as high-quality randomized trials or comprehensive systematic reviews.

Systematic reviews of randomized trials synthesize multiple studies and generally sit higher on the hierarchy, so they are not Level 2b. An individual RCT with a narrow confidence interval signals precision and typically represents higher-quality evidence, not Level 2b. Case-controlled studies are typically considered lower evidence than cohort studies, so they also do not fit Level 2b.