A systematic review uses an explicitly defined process to comprehensively identify all studies pertaining to a specific focused question, appraise the methods of the studies, summarize the results, identify reasons for different findings across studies, and cite limitations of current knowledge. Meta-analyses usually combine aggregate-level data reported in each primary study, which may provide a more precise estimate of the "true effect" than any individual study. However, the conclusions may be limited by between-trial heterogeneity, publication bias, or deficits in the conduct or reporting of individual primary studies.