The study, along with one of charter schools, was commissioned by the former head of the national Center for Education Statistics, Robert Lerner, an appointee of President Bush, at a time preliminary data suggested that charter schools, which are given public money but are run by private groups, fared no better at educating children than traditional public schools.

Proponents of charter schools had said the data did not take into account the predominance of children in their schools who had already had problems in neighborhood schools.

The two new studies put test scores in context by studying the children’s backgrounds and taking into account factors like race, ethnicity, income and parents’ educational backgrounds to make the comparisons more meaningful. The extended study of charter schools has not been released.

Findings favorable to private schools would likely have given a lift to administration efforts to offer children in ailing public schools the option of attending private schools.

An Education Department official who insisted on anonymity because of the climate surrounding the report, said researchers were "extra cautious" in reviewing it and were aware of its “political sensitivity.”

The official said the warning against drawing unsupported conclusions was expanded somewhat as the report went through in the review.

The report cautions, for example, against concluding that children do better because of the type of school as opposed to unknown factors. It also warns of great variations of performance among private schools, making a blanket comparison of public and private schools “of modest utility.” And the scores on which its findings are based reflect only a snapshot of student performance at a point in time and say nothing about individual student progress in different settings.