Among the more persistent arguments one hears in the debate over charter schools is that the “best evidence” shows charters are more effective. I have discussed this issue before (as have others), but it seems to come up from time to time, even in mainstream media coverage.
The basic point is that we should essentially dismiss – or at least regard with extreme skepticism - the two dozen or so high-quality “non-experimental” studies, which, on the whole, show modest or no differences in test-based effectiveness between charters and comparable regular public schools. In contrast, “randomized controlled trials” (RCTs), which exploit the random assignment of admission lotteries to control for differences between students, tend to yield positive results. Since, so the story goes, the “gold standard” research shows that charters are superior, we should go with that conclusion.
RCTs, though not without their own limitations, are without question powerful, and there is plenty of subpar charter research out there. That said, however, the “best evidence” argument is not particularly compelling (and it's also a distraction from the positive shift away from obsessing about whether charters do or don't work toward an examination of why). A full discussion of the methodological issues in the charter school literature would be long and burdensome, but it might be helpful to lay out three very basic points to bear in mind when you hear this argument.