Private-school vouchers are good for Florida's public-school system, according to a study to be released today by the Manhattan Institute.

The nonprofit conservative think tank found that students in low-performing schools threatened with vouchers improved much more than their peers in other schools on the Florida Comprehensive Assessment Test and the Stanford-9 math test.

Schools already facing competition from vouchers showed the greatest improvements of all five categories of low-performing schools, while schools threatened with the prospect of vouchers showed the second-greatest improvements, according to the study of 2001-02 school grades and standardized test results.

Gov. Jeb Bush, who made the grading system a priority of his first term in office, applauded the findings detailed in the report "When Schools Compete: The Effects of Vouchers on Florida Public School Achievement."

"Our reforms drive significant improvement in our schools by making them compete for students," Bush said in a statement. "This independent study validates results we've seen in schools across the state."

In Gadsden County, where Shanks High narrowly avoided becoming a voucher school last week when it won an appeal reducing its second F grade to a D, School Board spokeswoman Misty Cash said the threat of vouchers is indeed an incentive for schools to improve. She said the School Board continues to try to improve all the schools, but the school grades give the mission an added impetus.

"Having voucher schools is a motivation to have better schools and get off the voucher list," Cash said. "Does it improve our schools? We're always trying to improve our schools."

Researcher Marcus Winters said the report showed that it was the prospect of losing students - not the stigma of a D or F - that made failing schools better.

Some more skeptical

The Florida Education Association was skeptical about the findings. Spokesman Mark Pudlow said the Manhattan Institute has been outspoken in its support of Bush's A+ plan from the beginning.

"I think they are working hard to bolster what Gov. Jeb Bush is doing with vouchers in the state," Pudlow said. "We ought to take their conclusions with a grain of salt."

Pudlow noted that the research isn't reviewed by peers, as is common in the academic world. And when the institute's studies have been peer-reviewed in the past by "independent university types," many questioned the conclusions and the methodology, he said.

Winters said the institute is submitting the report to a peer-reviewed journal in hopes of getting it published. He said criticism is to be expected because the topic is controversial and many have strong opinions about it.

"We're very evidence-based here. We try to go wherever the evidence leads us," Winters said. "There are critics of our studies, just as we have criticized others."