That's taken care of in the study design. The population was all kids who applied to the lottery. And the treatment group wasn't those who actually attended the Montessori school, but those who were offered a place due to the lottery.
So I don't see how special needs would bias the results. If the lottery excludes those with special needs (either by design or due to self-selection) then there's no bias between control group and treatment group. If the lottery doesn't exclude but the enrollment decision is biased by special needs, then it doesn't matter because they use ITT and not enrolment.
Schools are not designed to calculate the actual cost on a per student basis.
Big ticket items like a dedicated SPED department, or a professional working 1:1 with a student can be accounted for. But if a special needs child participates in a standard class (which they do) and the standard teacher needs to do more than average work to accommodate them; that cost is not earmarked for that specific student. Once the bean counters see it, it is just "teacher salary", which gets averaged out across all the students.
Yeah, the intention to treat design is a particularly nice touch, not so common outside of biostatistics. They also compare the full cost of Montessori vs. plain ol', not the cost to the state, which could otherwise have given the Montessori schools (which are in wealthier neighborhoods on average) an unfair advantage if they have a lot of parents chipping in with donations and help. I've skimmed through the methods section and it does seem like they've gone to great lengths to allow for a fair comparison.
That doesn't necessarily mean the result will extrapolate, though. It seems plausible that teachers in Montessori schools are more motivated and knowledgeable than the average teacher and have made a conscious decision to teach in such a school. If every public school were to become a Montessori school, you would still get the cost savings (student-to-teacher ratios are higher in Montessori!) but you might lose that above-average enthusiasm and expertise and so the learning gains might not carry over. It's just really hard to know whether something might generalize in the educational sciences.
you might lose that above-average enthusiasm and expertise
yes, but montessori training can be done in one year (if you do it fulltime, my wife did i over multiple years 2 or 3 months each summer)), and it is entirely child focused. very different from traditional teacher training.
if we assume that every teacher starts their training with some amount of enthusiasm then the difference in enthusiasm and even more so in expertise should be minimal.
I've skimmed through the methods section and it does seem like they've gone to great lengths to allow for a fair comparison. That doesn't necessarily mean the result will extrapolate
Yes, I had exactly the same reaction. They appear to be presenting their work honestly, completely and clearly, so that other people have enough information to draw their own conclusions.
