Meta-analysis in the social sciences:

Meta-analysis is a class of statistical methods for combining the results from a series of studies addressing the same research question. These methods can powerfully test hypotheses that cannot be answered clearly with one or a few studies and eliminates the ambiguity that results from narrative reviews of a research literature or from counting the number of studies that support a particular conclusion. The usefulness of meta-analysis is demonstrated by reviewing instances in psychotherapy, education, and medicine that show how meta-analysis was used successfully to end acrimonious debate and reach a firm and lasting conclusion.