ABSTRACT
All meta-analytic approaches share one common characteristic. Namely, the results from the primary studies to be meta-analyzed must first be converted to a common metric. The two metrics used are the effect size, d, and the correlation coefficient, r. The Glassian-based meta-analytic approach (Glass, 1976; Glass et al., 1981) is probably the most popular approach to the metaanalysis of effect sizes. The Hedges and Olkin (1982) approach, a variation of the Glassian approach, has been presented as a more technically adequate form of the Glassian approach. However, Glass has restated the completeness of his original formulation (Glass & Kliegl, 1983). The meta-analysis of effect sizes is more common in research domains and disciplines that are characterized by the use of experimental or quasi-experimental research designs; certainly in those research areas that are characterized by identifiable experimental and control groups instead of correlational designs whose results are typically represented as correlation coefficients.
