But a recent study of standardized math scores in more than 1,300 public and private schools says the opposite may be true, according to Sarah and Christopher Lubienski, education professors at the University of Illinois at Urbana-Champaign.
Public school students from similar social and economic backgrounds tested higher in a national math achievement test than their peers in private schools, the Lubienskis say in an article to be published in the May issue of Phi Delta Kappan, an influential education journal.
They also are presenting their findings at the annual meeting of the American Educational Research Association (AERA), being held April 11-15 in Montreal.
"These results call into question common assumptions about public and private school effects, and highlight the importance of carefully considering socioeconomic differences in comparisons of school achievement," the Lubienskis wrote.
The achievement and survey data used in the researchers' study came from the 2000 National Assessment of Educational Progress (NAEP), the most recent annual assessment for which raw data were available to researchers.
The NAEP is considered the only nationally representative ongoing assessment of U.S. academic achievement.
The samples used in the research included more than 13,000 fourth-grade students in 607 schools (385 public and 222 private) and more than 15,000 eighth-grade students in 740 schools (383 public and 357 private).
Sarah Lubienski's first look at the data showed private schools outperforming publics overall in math achievement, which was no surprise and in line with most research. "But if you look at kids of equal socioeconomic class, the kids in public schools are outperforming the equivalent kids in private schools," she said.
She determined students' socioeconomic status (SES) by looking at six factors students were surveyed on in the NAEP assessment: eligibility for free or reduced lunch, eligibility for Title 1 funding, reading material in the student's home, computer access at home, Internet access at home, and the extent to which a student's studies are discussed at home.
For eighth-grade students, she also factored in the education level of the mother and father, as reported by students. (The item is not included in NAEP's fourth-grade survey.)
She then combined that data with school-reported data on the percentage of students in each school qualifying for Title 1 and for free or reduced lunch, to produce an SES variable for each school. The schools were then divided into fourths, or quartiles, representing low, low-middle, middle-high and high SES.
When the average math achievement scores of public and private schools within each quartile were compared, the publics came out higher in each quartile for both fourth and eighth grades, Sarah Lubienski said. In other words, when students were matched with their socioeconomic peers --whether poor, rich or in between -- the public school students came out with higher scores overall.
Sarah Lubienski said she was genuinely surprised by the results. After reworking the numbers several times, "we finally had to conclude that it's real," she said.
Chris, her colleague and husband, found the results intriguing in the context of his research on market-oriented education reforms, such as school choice, vouchers and charter schools.
"These results are significant because all the most prominent reforms right now assume that private schools do better, and that if you take a disadvantaged kid and give that kid an option to go to a private school, that will boost their achievement," he said.
The research at least provides grounds to question that assumption, especially since the data upon which it is based is more than two decades old, he said.
The researchers are careful to point out that the research does not follow individual students over time, or through any transitions between public and private schools. It cannot show, therefore, how individual students are affected in specific situations. The research, they said, should be seen as a snapshot in time that compares math achievement levels in a sampling of public and private schools, taking into account the background of their students.
"We can't make claims about the effects of schools on individual students," Chris Lubienski said, "but there's reason here to question the overall assumptions behind a lot of the private-market choice proposals being promoted right now."