There is good research on policy, but because so many individual policies are in play at any given moment, it is very difficult for policy researchers or policy makers to figure out the effects of anything.
In education, we also like to compare, and we usually don't like what we see. Comparisons can be useful in giving us a point of reference for what is possible, but a point of reference doesn't help if it is not seen as a peer.
Discussions about when generalization is most likely to take place are healthy and welcome. But they are not academic. America's schools are not getting better fast enough, and achievement gaps by race and class remain unacceptable.