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I once read a scathing refutation of tonsillectomies. One of the key pieces of evidence they based their argument on was the fact that children who had had tonsillectomies "still" had 1 throat infection per year, exactly the same as children who had not had tonsillectomies.
What they neglected to include was how many throat infections the tonsillectomy children were having before their tonsils were removed. Answer? Twelve per year, on average. Recipients of the surgery were a self-selected group, not random, and the fact that they were now average was near miraculous, not mundane.
The same is true with charter school students. They are by definition either self-selected, because they are for some reason angry enough with their default public school to leave, or selected by the district, because they are disruptive enough to get kicked out of their default public school.
You simply cannot compare performance of charter schools to public schools. You must compare the performance of charter school students before and after they joined the charter school.
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