Meta-analysis is a replicable and defensible method of research synthesis, aimed at a quantitative summary of quantitative findings across studies, with the focus on direction and magnitude of the effects across studies.

The first meta-analysis was performed by Karl Pearson in 1904, in an attempt to overcome the problem of reduced statistical power in studies with small sample sizes. Later, Glass coined the term "meta-analysis" to refer to "the statistical analysis of large collection of analysis results from individual studies for the purpose of integrating the findings" (Glass, 1976).

Meta-analysis is applicable to collections of research that

• are empirical, rather than theoretical
• produce quantitative results, rather than qualitative findings
• examine the same constructs and relationships
• have findings that can be configured in a comparable statistical form (e.g., as effect sizes, correlation coefficients, odds-ratios, etc.)
• are "comparable" given the question at hand