Lawmakers decided to hire an outside, neutral party to study the issue and inform debate.

The study, "Estimating Demand and Supply Response to Tuition Tax Credits for Private School Tuition in Utah," was done from an economist's point of view, very different from what's been presented so far.

It uses last year's bill, which would offer a tax credit for half of private school tuition up to $2,000, except for current private school kids. It attempted to account for demand, supply and cost variables by examining state and private school data and conducting 14 focus groups with public and home-school parents.

It ran several complex scenarios with different variables. All yielded different numbers.

"We know with 100 percent certainty the numbers we just gave you will not occur," Herzberg said. "We know you want us to tell you exactly what this will cost . . . (but) we can't."

Still, she noted, the analyses pointed to overall state savings.

Assume private school enrollment rises by just 1 percent over 13 years, or the time it would take for all students in private schools to be there under the tuition tax credits law. A middle-of-the-road estimate would put the first year's savings at $43.5 million if people took the full $2,000 tax credit. Year 13 would see a $2 million loss. But the state still would come out ahead — by $340.4 million in the whole time frame, that particular scenario shows.

The savings would be greater if private school enrollment grew by 3 percent over 13 years — totaling $1.3 billion.

The numbers partly hinge on marginal costs, preferred in economic analyses.

That's basically the cost of adding students to — or taking them away from — the public school system.

The study states Utah pays $8,675 in marginal costs per student, taking into account building costs, utilities, school supplies, personnel and other factors not tallied in Utah's per-student spending figures, which are significantly lower. Giving a tuition tax credit of $2,000 still would leave $6,675 in the system.

Still, there are risks, she said. Private schools might not attract many switchers, though Herzberg doesn't think that's much of a possibility. Schools also would have to adjust their spending on buildings and other costs or savings won't be realized.

Still, private school supply will increase with demand.

The study undoubtedly will be cited and re-cited as debate on tuition tax credits unfolds in the 2005 Legislature.

"It reaffirms what we (and others) have said . . . : Tuition tax credits are a financial boon to public schools," said Royce Van Tassell, spokesman for Education Excellence Utah, a school choice advocacy group. "This is pretty compelling evidence."

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E-mail: jtcook@desnews.com