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Comment by bsder | Hacker News Reader

> While overall charters and public schools compare relatively closely, both the 2009 and 2013 study found that charters did better for students in poverty. In addition, performance gap is growing over time:

It's not that hard to explain reversion to the mean. The schools in the lowest socioeconomic areas are generally, a priori, the worst of the bunch.

Consequently, just about anything will perform better. This is especially true if there is any ability to self-select (lottery is insufficient evidence as expelling "bad" students back to the public school is the same as self-selection).

When charter schools are compared in the suburbs, they generally perform worse than a public school.

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Almost all the evidence is based on lotteries and addresses attrition. Expelling bad students is a potential problem, but students are tracked across schools. Some students are lost, of course. However, the evidence in the KIPP study shows that attrition actually raises the scores of the lottery losers more than lottery winners. This is because winning the lottery encourages some bad students to stick with it.

I don't see the lottery as a bad thing. It lets kids ezcape from a bad environment. In a inner city public school, there is often too much peer pressure which forces the kids into poor performance or gang life.

The lottery is not a bad thing. And, I'm not even against non-lottery based selection for schools. They are allowed to do so.

The only issue is when trying to compare private school type X vs public school Y. Public school Y is required to take all comers ... no exceptions. If you want to claim that Private School X is better, worse, etc. you need to control for that because the worst students consume the most resource--even the expulsion of just a very small number of students can make a disproportionate difference.

I don't agree that this is reversion to the mean. If we saw it be the case that school districts with particularly good outcomes one quarter were more likely to have a decline in performance the next quarter independent of all observables, THAT would be reversion to the mean.

If there is a statistically significant difference in outcomes between charter schools and non-charter then I don't think we can dismiss it on the grounds you state.

But you did bring up an excellent point in the self-selection bias. We are potentially comparing apples and oranges if the charter schools get a different sort of student to teach.

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