Quantitatively reviewing the literature: the application of meta-analysis.

Meta-analysis has evolved as an alternative to the conventional narrative review. Its purpose is to statistically analyze a large collection of results from multiple studies in order to integrate the findings. There are three basic components of meta-analytic methodology. Inclusiveness involves the location of studies and decisions regarding which studies to include in the analysis. Quantification refers to the expression of such outcomes as effect sizes and the coding of study characteristics. Integrating outcomes involves the pooling of results from individual studies. Critics of meta-analysis have focused on aspects of the methodology, but careful meta-analytic procedure protects against most potential biases. The methodological and statistical rigor of meta-analysis pressures us to conceptualize our research more clearly and report results more completely.