In my ‘using data wisely’ work around Victoria I’m asked about calculating Hattie’s effect sizes. There are several good sites that show you how, here are some.
From Visible Learning
In the interests of perspective here I have included a colleagues of Hattie’s, Peter DeWitt’s column on effect size and several criticisms of Hattie’s work.
In short he suggests using effect size to calculate the effect of your current work before launching off into simply adopting some of his high leverage strategies.
I would add you also need to determine the level of fidelity (implementation level) there is to your current work as well. What everyone seeks, of course is high fidelity / high effect but if you get low effect be sure to check your implementation level first before taking something else on.