The original source to Caplan's blog post is a new book, The Rebirth of Education by Lant Pritchett. Two observations from Pritchett regarding education experience in the global context.
This isn't to say that across the board, private schooling is better than that available in government-run schools; in general, the evidence that private schools outperform government schools in well-functioning systems is weak. In the United States, where there has been the opportunity to do the most rigorous experimental studies, most researchers agree that the private sector edge in learning is nothing like a full effect size [1 standard deviation], almost certainly not even a tenth of an effect size, and some legitimately dispute whether the private sector causal impact is even positive.Caplan then points out the experience of Mexico.
Broader than just the success of specific interventions inside government schools is the observation that even in low-performing government systems one finds excellent schools, but also, even nearby and even operating under apparently exactly the same conditions, terrible schools... The problem is not that government schools cannot succeed, for in nearly all developing countries some of the very best schools are government schools. The problem is, as the LEAPS study authors emphasize, "when government schools fail, they fail completely"...
Case in point: In Mexico, "essentially all of the weakest-performing schools - those more than 100 points [2 standard deviations] below the average - are government schools."
Charter schools have been the great hope for cracking the demonstrable failure of many, usually urban, school systems. Lots of good will and good faith efforts and there are some very distinctive successes. But if you take all the charter schools together as a population, not just cherry pick the successes, then the results are much more mixed. But parents still love them (and government and teachers unions still hate them). Why?
My speculation had been along two lines of thought. 1) Parents love them because of the opportunity for greater involvement and influence than is usually possible in a bureaucratic and anonymous city system, and 2) charters are still operated within the broad guidelines and requirements of the school board - they have some latitude but not complete freedom, consequently there are likely some set limits to how much improvement might be possible.
Pritchett's work suggests an alternate or additional explanation. Perhaps the issue is not so much about average performance levels but rather of the standard deviations. I would wager that charter schools, like the private schools in Mexico, have a smaller standard deviation than the public schools. While the charter schools might not have much of an advantage on terms of the average of results that they do achieve, what they secure against is the risk of catastrophic failure (as exemplified in No books, no clue at city's worst school by Susan Edelman or the systemic teacher cheating scandals in Atlanta, Washington, D.C., Philadelphia and elsewhere).
So if your goal is to ensure that your child gets a good education, the average scores aren't all that different between public, charter and private schools. But if your goal is to ensure that your child avoids getting no education, then your choices are private and charter. Hence the sustained support for charters even though the academic results are only marginally better. Privates and Charters simply have a smaller measured standard deviation and that in itself can be immensely valuable.
The corollary question is: why do private and charter schools have a smaller standard deviation? I am guessing that it is a combination of greater accountability, greater transparency, greater competition and greater customer choice (a form of accountability). These are standard market place disciplines which are frequently absent from government systems. The tertiary implication is that public schools don't necessarily need charter schools so much as they need greater transparency, accountability, competition and choice.