ta-analysis provides a means of establishing the relevance of a potential moderator variable used to split studies into subsets. Large mean differences should appear, when meta-analysis one applies the procedure separately to each subset. When there are large differences in subset means, there is a corresponding reduction in within subset variation across studies. Thus, meta-analysis is capable of showing how much of the residual variation is due to artefacts (Vollet and Bousset, 2002).
Meta-analysis is a quantitative technique in which the results for each dependent variable in each of a group of studies are expressed as a difference between means for different groups divided by the standard deviation for the control group scores. These effect size scores, then, are averaged across studies, and the impact on effect size of several study characteristics is determined empirically.
Effect size represents the difference in the means of different groups, d
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