Everyone knows that private schools are better than public schools, right? Of course that’s a gross generalization — for instance, no one would claim that a third-rate private school is better than Weston High School — but surely it’s true in the aggregate. On the whole, the typical private school should be better than the typical public school.
These assumptions were challenged in a study by researchers at the University of Illinois, as reported in the Boston Globe a couple of months ago. The issue is more nuanced than simply comparing test scores. We all know that correlation doesn’t imply causation. Yes, the average SAT score from private schools is higher than the average from public schools, but that does not mean that a student who transfers from Weston High School to Podunk Academy is going to increase her scores. It doesn’t even mean that a student who transfers from Inner City High School to Phillips Academy will increase her scores. The extensive Illinois study (from 13,000 schools!) controlled for SES and found that the “obvious” conclusion was incorrect. In fact, students in public schools did better than those in private or charter schools when you ask the right question. It doesn’t matter much whether the average SAT score is better at school X or school Y; what matters is whether you as an individual student will be better off at school X or school Y. And that’s where the results are surprising.
Read the article, and find out why!