The latest fiscal impact review of Arizona’s scholarship tax credit programs estimates that they saved between $44 million and $186 million last year. The programs offer individuals and businesses dollar-for-dollar tax credits if they make donations to non-profit K‑12 scholarship-granting organizations. Those organizations, in turn, provide private school tuition assistance.

This is much higher than the savings estimate offered by the Arizona Republic last month, as the AZ Republic story linked above is quick to point out. I deal with the reasons for the discrepancy below, but first, here’s the crucial fact that the Republic has missed yet again: if the tax credit programs were significantly expanded, such as by raising the donation caps, the state would undeniably save many hundreds of millions of dollars annually. In fact, if the share of AZ schoolchildren participating in the program rose to just 40 percent, taxpayers would save billions of dollars a year — even if the size of the individual scholarships had to triple to achieve that result.

The Republic’s failure to report that inescapable and rather important fact does it no credit.

Now, on to the reason for the discrepancy in savings numbers. The body of the story hints at it: the Republic’s estimate assumed that private school enrollment would have been flat or increasing without the tax credit program, while the latest estimate does not.

As I pointed out at the time, the Republic’s assumption is demonstrably mistaken. Official AZ statistics show that enrollment in private schools peaked before the tax credit program had gotten under way, and had begun to decline as a result of rapid growth in the (tuition-free) charter school sector. So the Republic’s savings estimate was almost certainly too low.

As the author of the latest study admits, his assumptions about the true number of students who have migrated to private schools as a result of the program are speculative, but at least they are reasonable and not obviously erroneous, as the Republic’s were. In any event, the savings from a much larger migration to the private sector are not in doubt.