Level 1a is best described as which type of evidence?

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

Answer: (A)

Level 1a evidence refers to a systematic review that combines randomized controlled trials. This is the strongest level of evidence because randomized trials minimize bias through random assignment, and pooling results from multiple such trials increases statistical power and the precision of the estimated effect. The systematic review process adds rigor by using predefined search strategies and inclusion criteria, which reduces bias in selecting studies and extracting data. When a meta-analysis is included, it provides a single overall estimate and helps assess consistency of results across different populations and settings. While an individual RCT can be informative, Level 1a gains its strength from synthesizing multiple high-quality trials, addressing variability and improving generalizability. Lower on the hierarchy would be systematic reviews of observational studies or expert opinion, which carry more risk of bias.