A statistical method that combines results from many individual studies into one overall estimate — the most powerful form of clinical evidence when done well.Requires that pooled studies address the same research question with comparable designs, populations, and outcomes. When studies are methodologically heterogeneous, pooling them produces a statistically precise but potentially misleading estimate (garbage in, garbage out). The Cochrane meta-analysis of thymosin alpha-1 for HBV found insufficient evidence for overall survival benefit — a meaningful negative finding from a systematic synthesis of available data, not just an absence of studies.