As Pennsylvania's debate over school vouchers kicks into high gear this week, one of the issues that proponents and critics can be expected to debate most fiercely is the impact of vouchers on public schools.

Voucher advocates contend that public schools will be forced to improve if they have to compete for students and the taxpayer money that goes with them.

Critics counter that vouchers can only cause harm by siphoning students, money, and possibly some attention away from the public schools.

While studies are relatively scarce, the early opinion among researchers appears to be that vouchers have done little, if any, harm to student achievement in public schools and in some cases have spurred small improvements on standardized-exam scores in public schools.

One study of the Florida voucher system found that standardized-test scores in public schools improved slightly, for example.

Studies in Milwaukee reported conflicting results on test scores in public schools, but found that vouchers saved state taxpayers money while yielding high levels of satisfaction among parents who opted to take their children out of public schools.

For those considering adopting voucher programs, "there's very little to be afraid of," said David Figlio, a professor of education and economics at Northwestern University who has studied Florida's vouchers, which are funded by corporations that receive a tax credit in return.

Still, he cautioned against believing that vouchers were the answer to reforming American public education.

"Anyone who thinks voucher competition will fundamentally change the way public schools do business, my take is you need to keep looking," Figlio said. "We're not seeing the magical pill that's going to turn the U.S. into Finland."

Voucher programs allow students to attend private or parochial schools with the aid of public dollars or, in some cases, funding from businesses.

Specifics vary in existing programs - including those in Florida; Milwaukee; Washington, D.C.; and Dayton, Ohio - but most are limited to low-income students.

Advocates in Pennsylvania are hopeful that with a new governor supportive of school choice, the state could adopt Senate Bill 1, which would create a government-funded voucher program for low-income students, before the end of this school year.

The Senate Education Committee is scheduled to discuss the bill Wednesday.

The state already has the Educational Improvement Tax Credit, which encourages businesses to provide scholarship money in exchange for tax breaks. Statewide, 44,000 students a year use such funding to attend private or parochial schools, according to the REACH (Road to Educational Achievement Through Choice) Foundation, a grassroots coalition that favors school vouchers.

In New Jersey, school-choice advocates are optimistic that a bill to create scholarships funded by corporate tax credits, modeled on Pennsylvania's EITC program, will make it to Gov. Christie, who supports the measure.

Proponents say the improvements in public schools caused by vouchers, even if small, are significant.

"The improvements in achievement that several researchers have found in careful studies aren't large, but they are positive," said Patrick Wolf, who teaches in the department of education reform at the University of Arkansas College of Education and Health Professions.

He warned that the design of a voucher program could have a significant impact on the outcome.

Jay Greene, who heads Wolf's department, said his study of the Washington system had found that it did not improve the public school system, probably because the program ensured that public schools that lost students did not lose funding.

"It seems to be necessary to have some money on the line," Greene said. "If public schools are completely held harmless, it seems to undermine the incentive to improve in response to competition."

While vouchers have had at most a modest positive effect on test scores in public schools, he added, the programs in question have been so limited that they didn't create much competition.

"If you took a very low dose of aspirin, you would get a very small amount of pain relief," Greene said.

Figlio said he had started out skeptical that vouchers would have any effect on public schools. But he found that Florida's program slightly increased test scores at public schools, and that the improvements were largest at schools in areas with many private schools that students could choose.

"They're small, but they're educationally meaningful, and they're persistent," Figlio said.

Competition to improve public schools is only one argument advanced by voucher proponents. Many of them argue that the programs save taxpayer money because the vouchers typically cost less than the cost of educating a child in a public school.

The early research bears out that hypothesis, said John Witte, a professor of public affairs and political science at the University of Wisconsin, who is the principal investigator for a research team studying the Milwaukee program, which began 20 years ago and is the oldest and largest of its kind in the country.

In Milwaukee, Witte said, parents who used vouchers to send their children to private or parochial schools also tended to report much higher satisfaction with those schools despite little improvement, on average, in their children's academic achievement.

He said there also was little evidence in Milwaukee that the highest-achieving students used vouchers to leave the public schools, as some voucher critics have feared.

Witte and other researchers speculate that instead of the most academically oriented or motivated parents using the vouchers, those whose children left the public schools were for one reason or another very dissatisfied with the public schools.

Dennis Epple, a professor of economics at Carnegie Mellon University in Pittsburgh who has studied school vouchers, urged Pennsylvania policymakers to require careful data collection to allow independent researchers to examine the impact.

Epple, who recently cowrote a 180-page review of the research on school vouchers, said more research needed to be done.

"The jury is still out on vouchers," he said. "I think the Milwaukee case has encouraging signs that if you stick with it and have a sustained program over a long period of time, I think it does foster some beneficial competition."

Contact staff writer Adrienne Lu at 215-854-2624 or alu@phillynews.com.