[themaninthedark]

So while it is randomized in the terms of who they chose to participate, the students did attend already existing schools. This could lead to selection bias from the participates, as the schools themselves are located in wealthier areas as that is where their clientèle are.

Lower income families may not have been able to take advantage of the lottery due to distance constrains thus self-opting out.

I have not read the study methodology details, the schools may have been chosen to avoid this problem but just wanted to point out that just because something say "random lottery" it may not be.

[dmurray]

But the control group was families who did opt in and who lost the lottery.

[cma]

Still the ones who got in would be with more other students who opted in, the ones who didn't get in would be with more students who didn't opt-in to the lottery.

You would need to have a second group of those who lost the lottery and were all put into the same non-Montessori school with no others who didn't opt-in maybe.

[rahimnathwani]

Oh you mean it could all be peer group effects?

[cma]

Yeah but even then teachers that opt in to train in Montessori might just be better teachers, and converting a whole school system to Montessori, training everyone, might not have as good results.