Gene V Glass coined the term meta-analysis in his 1976 presidential address to the American Educational Research Association. Glass bemoaned the state of educational research, and scientific and evaluative enterprises more generally, left fragmentary and incoherent through failure to integrate conclusions across the burgeoning number of studies and disciplinary specialties. What if, he asked, we applied statistical analysis to groups of studies just as researchers and evaluators statistically analyze groups of humans? What if we could compare study conclusions on a common metric, determine not only the likelihood but also the strength of an intervention's effects, and test whether different intervention conditions influence effectiveness? Glass argued that conventional statistical techniques could be used to create a higher order analysis beyond (“meta”) the analysis presented by ...