First, I love stats and especially ones that can shape my theory to guide my practice.  Hattie, some could argue is one of the gurus of educational stats because he has been looking at such vast amounts of data through his meta-analysis.  The article I cited at the bottom explained a main argument against Hattie’s grouping and rankings, which can be found here: http://visible-learning.org/hattie-ranking-influences-effect-sizes-learning-achievement/

It would be interesting for a researcher to delineate these two types of studies that could be derived from Hattie’s findings:

APPLES: “The truth is that Hattie sometimes uses ‘effect size’ to mean ‘as compared to a control group’” (EC, 2012)
One group is a control – the other is the treatment

ORANGES: “and other times uses it to mean ‘as compared to the same students before the study started.'” (EC, 2012)
There is no control, but an effect size can still be determined.

It would be further applicable if the studies were broken down by age, skill level, or grouping.
For example, primary school children, middle school, and high school.

Reference:
EC, 2012 – http://literacyinleafstrewn.blogspot.ca/2012/12/can-we-trust-educational-research_20.html