OK, I am kind of annoyed. I've been going through John Hattie's impressive book, Visible Learning, trying to make sense of it, and I'm confused on a basic point. Here I am reading this long book that is all about comparing effect sizes, and I realize that I don't know exactly what Hattie means by "effect size." If a program has an "effect size" of 0, does that mean that the the kids in the program didn't learn anything, or does it mean that they didn't learn anything more than the kids in the control group? At different times in the book, it seems to mean different things. It is hugely annoying that Hattie doesn't make this clearer. He claims to have spent 15 years writing the book, and he can't even be clear about the most basic possible element of his work? Grrr.
Of course, part of what makes me so annoyed is that I am aware that the problem is probably partly mine as well as Hattie's. If I were smarter, maybe I'd be able to figure it out. But I can't help suspecting that maybe "effect size" means different things for different studies, and that this may have some effect on Hattie's conclusions.
Update (12/16): OK, it's not my problem, it's Hattie's. More later, but this is just ridiculous. The state of educational research is just pathetic. This guy is some kind of big shot, and his book is supposedly a masterwork fifteen years in the making, and he's making various basic errors. I didn't catch them at first because I'm an English teacher, not a statistician, but it didn't take me THAT long, either. How is it that people continue to take him seriously? I'll explain in a bit more detail in another post when I have a free half hour or so.