But some economists say the Utah State University study is not as bulletproof as advocates and researchers maintain.

The researchers found tuition tax credits could cost public schools $179.3 million over 14 years or save them up to $1.2 billion, depending on the appetite for private schooling set off by Utahns who bite on the tax break.

"We do not think this is a serious financial risk for the state," USU professor Roberta Herzberg said.

Even so, the huge range is not reliable enough for a policy decision, said Clive Belfield, an economist and associate director of the National Center for the Study of Privatization in Education at Teachers College at New York's Columbia University.

"The report is incredibly long, highly complex and, unfortunately, very inconclusive," he said. "It's not clear enough to tell me whether I'm losing money or saving money. It's not the definitive answer. Certainly, the public is not well served by this document."

Back in July, the Legislative Management Committee hired Herzberg and fellow USU professor Chris Fawson to answer the question: What kind of economic impact would public schools face if parents were allowed to claim income-tax credits for switching their children from public to private schools?

In Utah, income-tax revenue is the largest source of public-school funding.

Herzberg, a political scientist, and Fawson, an economist, built a computer-based model that crunches several variables to predict whether decreases in public-school enrollment and income-tax revenue helps or hurts public schools.

Variables include the amount of the tax credit ($1,000 or $2,000 per student), the number of students the tax credit would induce to jump to private schools, the supply of private school slots to meet the new demand, and the costs public schools would eliminate for each student who switches.

Calculating costs: Critics of the study point first to the researchers' estimate of marginal cost, a term they use to describe how much the state will save for each student who switches to private schools.

The authors use a figure of $8,675, which far exceeds the $6,500 or so total in per-pupil funding (the lowest in the nation).

They say it's higher because it includes everything from books and desks to buildings and teachers. For example, they say, a district would have to build a new school if its enrollment surged by 300 students.

Some don't buy that rationale.

"It's very worrying that the children who switch will save you more than the average kid," Belfield said. "You always assume that, yes, you're going to save if a kid moves out, but you're not going to save on fixed costs. You're not going to sell the desk the kid is going to be at."

Fawson counters that - as students leave the system - districts could redraw boundaries to consolidate schools. That could result in building closures and greater efficiencies.

But there may be another problem with the authors' figures, which were revised downward earlier this month.

Some students cost more than others, and there is no way to know which ones will stay and which ones will go, said Joydeep Roy, an education economist with the Economic Policy Institute, a Washington-based think tank that focuses on low-income and middle-class families.

"The marginal cost of educating a student in the public school that they estimate is very large, too large to be plausibly true," he said. "And if it is the easier-to-educate, richer and middle-class families who opt out of the public school system using tuition tax credits, the savings in cost to the public schools would be even less."

The seminary question: Perhaps the most important - and elusive - factor in determining economic impacts of tuition tax credits is an accurate projection of the number of children who actually would leave public schools to attend private schools.

The USU study generated low-, medium- and high-demand scenarios. The researchers say high demand is plausible - perhaps likely - because of the recent boom in charter-school enrollment, an increase in private schools and changing demographics.

But skeptics wonder why a tax credit would change Utah's historically low private-school enrollment. Less than 3 percent of Utah children attend private schools, compared with 12 percent nationwide.

Herzberg predicts the parents who take advantage of tuition tax credits would be evenly split between those seeking religious education and those with children struggling in public schools.

"The most likely parent is the parent who has had failure in the public school for one reason or another," she said. "Another set of parents is looking for a moral or disciplinary arrangement that is not available in the public school system."

Nationally, religious schools dominate the private-school landscape: More than 4 of every 5 private-school students attended sectarian schools in 1999-2000, the most recent data available from the National Center for Educational Statistics.

Absent from the USU study was an examination of how LDS seminaries - found on church property near most public secondary schools - might already address demand for religious education.

According to The Church of Jesus Christ of Latter-day Saints, 75,175 ninth- through 12th-graders attend seminary in Utah. That's more than half of the 143,369 students attending those grades in the state's public schools.

If religion is the predominant reason parents pick a private school, wouldn't those seminary classes fulfill many families' needs?

"The key influence on demand for private schooling is religion, and the authors barely touch this," Belfield said. "It would take a lot of non-Mormon kids entering [public] school to get a reasonable amount of tension and friction" to draw them to private schools.

Herzberg says this could happen as Utah grows more diverse.

Good science, bad policy? All in all, the report concludes that tuition tax credits could eliminate millions of dollars in public-school expenses or millions of dollars in public-school funding.

The authors predict it will bolster the school system's bottom line.

But Belfield says it's just guesswork.

"They spend a lot of time on the numbers, and the numbers are pretty hopeless," Belfield said. "It's not their fault. It's just that the numbers aren't there to do solid analysis."

The USU researchers acknowledge that any one of those scenarios could play out if legislators approve tuition tax credits when the 2005 general session begins next month.

"We offer the entire range [because] we don't know exactly what level it will be," Herzberg said. "It may be great policy even if [tuition tax credits] cost the state a lot. And it might be terrible policy if it saves the state a billion dollars. . . . It's a much fuller debate than this study."

rlynn@sltrib.com

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Tribune reporter Peggy Fletcher Stack contributed to this story.