Private School Advantage Entirely Due to Self-Selection

A study of 4th and 8th graders finds that on average public schools might be slightly ahead of private schools when it comes to reading and math. From the NYTimes:

The report, which compared fourth- and eighth-grade reading and math scores in 2003 from nearly 7,000 public schools and more than 530 private schools, found that fourth graders attending public school did significantly better in math than comparable fourth graders in private schools. Additionally, it found that students in conservative Christian schools lagged significantly behind their counterparts in public schools on eighth-grade math.

This study cannot be taken to mean that public schools are better than private schools. The report itself warns against such comparisions, because of the great variation in performance within private schools themselves. But it does show that on average, when taking into account "social and economic backgrounds" public schools do not fall behind private schools. This means the higher test scores seen in private schools can almost entirely be attributed the social and economics class of the students themselves.