Full Question: In a recent Hattie article called, "The Applicability of Visible Learning to Higher Education” collective teacher efficacy is listed as a 1.57 effect size. I have no doubt that the effect size is that high. What I request is evidence. There is no evidence listed in that article. There is no evidence in the Eells dissertation (which is often quoted). There is no evidence in Donohoo’s book on collective teacher efficacy. The greater the claim, the greater the need for evidence. Can you find for me the evidence that Dr. Hattie uses for that claim?
Answer: “Using random-effects modeling, which assumes significant between-study variance, the weighted average effect size for r-mean is 0.598, with a 95 percent confidence interval of 0.542 - 0.649. When the same meta-analysis was repeated with the outlier (Antonelli, 2005) removed, homogeneity testing indicated that there was less variance to explain: Q=38.198 (df=24, p<0.03). The weighted average effect size of r-mean (outlier removed) was 0.617, with a 95 percent confidence interval of 0.571 - 0.659. And this is a correlation so it needs conversion to an effect size using d = 2r/ (sqrt (1-r*r)) = 1.196/.801 = 1.49. Hope this helps” (John Hattie, personal communication, May 23, 2017).
NB: The effect size for collective teacher efficacy (as of 05.30.19) 1.39.