On the Public-Private School Achievement Debate

Abstract

On July 14, 2006, the U. S. Department of Education's National Center for Education Statistics (NCES) released a study that compared the performance in reading and math of 4th and 8th-graders attending private and public schools. Using information from a nationwide, representative sample of public and private school students collected in 2003 as part of the ongoing National Assessment of Educational Progress (NAEP), the NCES study reported that the performance of students attending private schools was superior to that of students attending public schools. But after statistical adjustments were made for student characteristics, the private school advantage among 4th-graders was reported to give way to a 4.5-point public school advantage in math and school-sector parity in reading. After the same adjustments were made for 8th-graders, private schools retained a 7-point advantage in reading but achieved only parity in math. In this paper, we argue that NCES's measures of student characteristics were flawed by inconsistent classification across the public and private sectors and by the inclusion of factors open to school influence. Utilizing the same data as the original study but substituting better measures of student characteristics, improved, alternative models identify a private school advantage in 11 out of 12 public-private comparisons. In 8th-grade math, the private school advantage varies between 3 and 6.5 test points; in reading, it varies between 9 and 12.5 points. Among 4th graders, in math, parity is observed in one model, but private schools outperform public schools by 2 and 3 points in the other two models; in 4th-grade reading, private schools have an advantage that ranges from 7 to 10 points. However, although the alternative models constitute an improvement on the NCES model, no conclusions should be drawn as to causal relationships from these or any other results based on NAEP test scores, because they are too fragile to be used for such purposes. Inferring causality from observations at one point in time is highly problematic.