A New Look at Public and Private Schools: Student Background and Mathematics Achievement

The claim that private schools do a better job of educating their students than public schools -- an argument central to the push for market-style reforms -- is rarely questioned. But do the data back it up? The authors examined fourth- and eighth-grade NAEP mathematics achievement in over 1,000 public and private schools to arrive at a surprising answer. A RECENT report of mathematics results from the National Assessment of Educational Progress (NAEP) highlighted this "major finding": "Public- school students scored lower on average than non-public-school students at both grades 4 and 8. "1 Of course, this finding is nothing new. Indeed, it is part of the common wisdom in the United States that private school students outscore public school students on standardized tests. Furthermore, studies have suggested that this is true even when researchers account for the fact that the enrollment at public schools differs from the enrollment at private schools. This belief is based, in part, on past studies involving the 1980 High School and Beyond dataset that found that private schools are more effective than public schools at boosting student achievement, including that of disadvantaged students.2 These studies of test performance, which controlled for some potentially confounding variables such as socioeconomic status (SES), affirmed widespread assumptions about the superiority of private schools. These assumptions, in turn, have influenced recent reform efforts promoting various forms of privatization of public schools, including the No Child Left Behind Act, which makes use of a variety of private sector sanctions for "failing" public schools. However, the seminal studies on private school effects are becoming dated, with most based on a sample of students who began school over a generation ago. In addition, most of these studies were limited to high school.3 Some used rough measures of SES that may not have been sufficiently sensitive to the differences in populations served by public and private schools. Given the current interest in choice and privatization, it is an important time to examine current evidence relating to achievement in public and private schools. While undertaking a broader study of mathematics instruction and equity, we became intrigued by an unexpected finding: when controlling for private school status and student background variables in our statistical models, we saw that mathematics achievement in public schools actually appeared higher than that in private schools.4 We decided to do a special substudy focusing specifically on achievement differences between public and private schools. Using a powerful SES variable created for the broader study, we were able to more carefully examine the question of whether the widely assumed "private school effect" is due more to the population of students served than to institutional effectiveness. The study focused solely on student achievement in mathematics -- a subject generally thought to be less influenced by family background and more influenced by institutional effects than other school subjects such as literacy. The Role of Family Background NAEP has been an important tool for monitoring trends in U.S. student achievement. It is the only nationally representative, ongoing assessment of U.S. academic achievement, measuring student performance at fourth, eighth, and 12th grades in mathematics and other subject areas. Our study used fourth- and eighth-grade data; eliminating 12th- grade data removed the issue of high school dropouts, which could bias our samples. Furthermore, the focus on students in the earlier grades reduces the likelihood that the students in the sample had transferred between public and private schools. We analyzed achievement and survey data from the 2000 Main NAEP mathematics assessment,5 which we obtained from a restricted-use CD-ROM. (When the study was conducted in 2004, the 2000 assessment was the most recent for which raw data were available to researchers. …