Analyzing raw data from the 2000 National Assessment of Educational Progress for 28,000 fourth- and eighth-graders representing more than 1,300 public and private schools, Mrs. Lubienski, whose research focuses on equity issues in math education, was surprised by what she was seeing. When children of similar socioeconomic status were compared, the public school children scored higher. She called in her husband, who studies school choice and privatization, to help interpret the results.
When the students were divided into four socioeconomic groups, the difference between public and private school math scores was 6 to 7 points for fourth-graders in each group, and 1 to 9 points for eighth-graders. Not a large difference, says Mrs. Lubienski - more "small to moderate."
The crux of the study isn't so much to suggest that public schools are outpacing private schools as to call into question a common assumption: "It's more: 'Wow, this flies in the face of what we thought - that private schools do better than public.' "
Some, however, are skeptical. Any time studies produce "counterintuitive" results, they should be carefully examined, says Joe McTighe, executive director of the Council for American Private Education (CAPE).
Without having seen the Lubienskis' research, he points out that raw scores have typically shown the country's 6 million private school students, who make up 11.5 percent of US schoolchildren, outperforming public school students.
But others say the results are not that surprising.
"We would conclude, on the basis of perhaps 15 years of research, that there's nothing magic about privatization," says Henry Levin, a professor of economics and education at Columbia University's Teachers College in New York, and director of the National Center for the Study of Privatization in Education.
For the most part, he says, the academic benefits of attending a public or a private school have been relatively small, when compared in studies.
Certainly the stubborn gap between the academic achievement of white and minority students across the board is greater than the gap between public and private school performances.
That's a salient point for those who oppose the idea of privatization as a cure for the ills of US public education. Unless private schools have a measurably better track record than public schools, it's hard to argue that privatizing education will necessarily boost performance for all students.