Systematic review

A systematic review is a structured synthesis of the evidence on a defined question, using an a priori protocol that specifies eligibility criteria, search strategy, data extraction, and risk-of-bias assessment. Reporting typically follows the PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) statement, and protocols are often registered prospectively in PROSPERO. A systematic review may or may not include a meta-analysis: when clinical and methodologic heterogeneity preclude pooling, the synthesis remains narrative but still transparent and reproducible. Systematic reviews are distinguished from traditional narrative reviews by their explicit methods and pre-specification, which reduce selection and confirmation bias. The 2017 NASEM cannabis report relied heavily on prior systematic reviews to structure its evidence-grading conclusions, and Whiting et al. (2015) screened 23,754 records against prespecified criteria to identify 79 eligible trials. → See also: Meta-analysis, Cochrane review.