re: just wondering about this study.
Me too. There are so many variables in something this complicated and I have to wonder about how many of them were taken into consideration or even identified. I am not putting down the study, I think it's one that needs to be done and done right. But as you point out something as simple as the demographics of those whose kids are being studied can really affect the outcome.
My wife has been in the preschool setting for more than 30 years and she can tell you in no uncertain terms the damage that is done to a child's chances of success by being in a care setting from birth through high school. And she is in a center that's in one of the best neighborhoods in our county. She has parents who drive up in their $60,000 cars, drop off their kid(s) at 8:30 AM and don't pick them up until 5:30 PM. Their day is not spent working. These children are desperate for the time and attention of a parent.
I used to work in a day care; the hours between 3 pm (after naptime) and 6 pm (pickup time) are especially sad; everyone just wants to go home, children and teachers alike.
This study had a number of counter-intuitive findings: it found larger class sizes and less experienced teachers are correlated with higher math achievement!