The researchers claim a recent U.S. Department of Education study of the same test results was flawed.

The government study, which fanned the flames of the school voucher debate when it was released last month, said public school students did roughly the same as, and in some cases better than, private school students in fourth and eighth grades.

Both studies compared the scores of fourth- and eighth-grade students from nearly 7,000 public schools and 530 private schools on the National Assessment of Educational Progress test.

The Harvard researchers said the government study was skewed toward schools receiving federal funding for students from low-income families and receiving specialized education services.

"When you use participation in federal programs as a measure of a student's family background, you undercount the number of disadvantaged students in the private sector," said Paul Peterson, a professor of government and one of the study's authors.

By contrast, Harvard's study gave a more accurate picture of student performance in both public and private schools, Peterson said.