A Harvard professor responded to an New York Times article critical of his study on school vouchers yesterday, writing a letter to the Times defending his work and suggesting that the article exaggerated the criticism.

Last Friday, the Times reported that Mathematica Policy Research, a think tank that provides academic data, took the unusual step of questioning the conclusions of a school vouchers study by Shattuck Professor of Government Paul E. Peterson.

Yesterday, Peterson and his colleagues wrote a joint letter to the Times suggesting that the article generated controversy where there was none to be found.

"Your story suggests a level of disagreement among researchers that simply does not exist," the authors write.

The two-year study found that, in three different cities, vouchers had a statistically significant effect in raising the test scores of black students although no corollary effect was seen for other racial groups.

Peterson's findings were quickly picked up by voucher supporters and lauded as some of the first genuine evidence in favor of the use of vouchers.

But last week's press release by Mathematica called the validity of the study into doubt, questioning Peterson's use of data. The release prompted Kate Zernicke's article, "New Doubt is Cast on Study that Backs Voucher Effects," in Friday's Times.

The release claimed the New York segment of the study shows "no overall difference in test scores between those who were offered vouchers and those who were not."