Abstract :
Meta-analysis is a statistical method that can appear complex and intimidating
at first glance (Rosenthal & DiMatteo, 2001). But at its heart, a meta-analysis
calculates the mean and variance of a set of numbers. The numbers are not individual
scores, however, as researchers are accustomed to averaging, but instead
are statistics reported across studies within a particular research domain, such
as a set of study correlations, standardized mean differences between groups,
or odds ratios. For at least 150 years, scientific researchers have engaged in
the practice of averaging effects found across a set of studies or scientific
observations; however, meta-analysis has developed relatively recently as a
formalized statistical method for doing so (for information on the development
of meta-analysis, see Borenstein, Hedges, Higgins, & Rothstein, 2009; Hunter
& F. L. Schmidt, 2004; for early works on meta-analytic methods, see Cooper &
Rosenthal, 1980; Glass, 1976; Hedges & Olkin, 1985; Rosenthal, 1978; F. L. Schmidt
& Hunter, 1977).
