Which makes one wonder as to what the justification was for aggregating the data in the way it was aggregated, or aggregating it at all in the first place, othe

Which makes one wonder as to what the justification was for aggregating the data in the way it was aggregated, or aggregating it at all in the first place, other than to contrive a statistic to argue for whatever policy position the authors already support. If the honest purpose of the study is to compare the results of various educational methodologies, I don't see that there'd be a useful unit of analysis any larger than an individual school, and even that might be too coarse. It's true that American states have de jure authority over public schools, and that there isn't any singular "American educational system" to speak of, but even within the individual states, there's still a great deal of variation from district to district and from school to school. Plus, there are plenty of schools outside the conventional public education systems, whose methodologies aren't determined by policy from any government.