I don't think that Hattie's work faithfully represents the research. For example, let's go down the rabbit hole on his 'research' on class size.
The major paper that Hattie uses was by Glass and Smith from 1979. The results are graphed here: https://i.imgur.com/Qrom8X7.png and here: https://i.imgur.com/xCGKy0P.png
The trend and difference between good and poor research are clearly displayed. Their conclusion:
> The curve for the well-controlled studies then, is probably the best representation of the class-size and achievement relationship...
> A clear and strong relationship between class size and achievement has emerged... There is little doubt, that other things being equal, more is learned in smaller classes.
Hattie stated that:
> Glass and Smith (1978) reported an average effect of 0.09 based on 77 studies...
Glass and Smith didn't report this at all. When contacted Professor Glass replied:
> Averaging class size reduction effects over a range of reductions makes no sense to me.
> It's the curve that counts.
> Reductions from 40 to 30 bring about negligible achievement effects. From 20 to 10 is a different story.
> But Teacher Workload and its relationship to class size is what counts in my book.
Other researchers in this field on this topic:
... the list goes on and it goes on for every single topic.
Oh, Glass and Smith invented meta-analysis as a research methodology and as such defined most of the protocols for its success.
> The result of a meta-analysis should never be an average; it should be a graph. - Glass and Smith