Meta-analysis at 20: retrospect and prospect.

It has been 20 years since Gene Glass reinvigorated research integration through the formalization of meta-analysis. This article explores the nature of meta-analysis by placing it within the context of research synthesis. The methods of meta-analysis are described briefly and compared and contrasted with other forms of research integration. In addition, the findings from several meta-analyses are provided to show the advantages of quantitative review methods. Finally, criticisms of meta-analyses are analyzed, as are several unanswered questions about the direction of meta-analysis.