I understand that research into something as complicated as education can't control all variables. But I can immediately think of numerous other variables - what about the other children at the schools backgrounds? what about the level of funding the schools received? the level of training of the teachers?

I think Montessori and Steiner are far more dogmatic than sense dictates - but stay this way, despite the negatives, in order to give their system a magical quality. Determining the quality of a school is hard. So people are willing to cling to a belief because they don't know any better.

Just one example of negative; Montessori doesn't use imaginative play. It's been shown to be a great tool to give kids better concentration spans. But the writings of Montessori are against it and the dogma has to be followed.