Cumulative research knowledge and social policy formulation: The critical role of meta-analysis.

For many years, policymakers expressed increasing frustration with social science research. On every issue there were studies arguing for diametrically opposed conclusions. Methods of meta-analysis that correct for the effects of sampling error have shown that almost all such conflicting results were caused by sampling error. Furthermore, the effects of sampling error are greatly exaggerated by using significance test methodology. In many areas, meta-analysis has now provided dependable answers to the original research questions. Meta-analysis is now increasingly being used by policymakers, by textbook writers, and by theorists to provide the basic facts needed to draw both practical and explanatory conclusions. Sophisticated meta-analysis procedures are now used to correct for the effects of other study imperfections, such as measurement error, range restriction, and artificial dichotomization. In domains where the data on artifacts are available, the effect sizes in necessarily imperfect studies have been found to be considerably understated. Path analysis can be applied to the findings from meta-analysis to yield improved causal analyses that result in both explanation of results and improved generalization of