Proponents of tuition tax credits initially hailed the report as substantial evidence that Utah should adopt tuition tax credit legislation in 2005. But that judgment proved to be a bit hasty.

The report is so long and complex that it took time to digest, but when the figures - which had to be revised substantially when researchers found a "glitch" in their analysis - were better understood, the evidence appeared much less substantial.

The report's conclusions are based on too many "ifs," the most important of which is the unknown, and probably unknowable, number of students who would leave public schools for private schools if tuition tax credits of $1,000 or $2,000 per student were available.

Which students might leave is another variable that would have a tremendous effect on the outcome, because some students cost more to educate than others.

The report uses an inflated figure, $8,675, for the potential savings schools would realize per student. That number far exceeds the per-pupil figure of $6,500 used in state budgeting for education. The higher figure wrongly assumes that fixed costs - buildings, equipment, utilities, salaries - would be reduced.

However, it would take many hundreds of students leaving public schools before physical facilities could be consolidated, and there is little evidence to suggest that sufficient numbers of Utah parents would prefer private schools.

On the contrary, Utah historically has low private-school enrollments - 3 percent compared with 12 percent nationwide. Generally, religion is an important, if not the most important, reason parents choose private schools. In Utah, where The Church of Jesus Christ of Latter-day Saints is the predominant religion, students can attend LDS seminaries located adjacent to most public secondary schools, fulfilling the demand for religious instruction.

The report offers a broad range of possible scenarios if tuition tax credit legislation were adopted - too broad to serve as a sound basis for such a far-reaching change in policy. At best, public schools might realize some savings, but lose vital public and legislative backing. At worst, public schools that are already overcrowded and underfunded could lose millions more, and education for the vast majority of students who remain would suffer.

The USU researchers acknowledge that they could only guess at which scenario is most likely to occur. Education policy must not be based on guesswork.