A number of factors have been included in this model to try to make it more realistic. First, the increase in the relative affordability of private schools will also likely boost the rate of increase of private school tuition. As noted above, private school tuitionjust like public school "per pupil expenditures"does not include the full cost. It is easy to anticipate that, should a tuition tax credit program go into effect, some of those costs currently being subsidized by churches, religious institutions, or parents volunteer work would no longer be subsidized.97 Thus, we have assumed that the average private school tuition grows at 4.5 percent per year, 50 percent faster than the 3.0 percent growth assumed for the public school per-pupil guarantee to account for this effect.
Secondly, we have also noted that behavior does not change overnight. Parents with children in traditional public schools are not likely to pull them all out or move them around in the first year the tuition tax credit is available. Thus, we have smoothed the migration behavior slightly by including in the model an adjusted desired migration factor, which indicates the likely migration that would occur with the implementation of the tuition tax credit plan. That smoothed migration factor starts at 17 percent in 1999, tapering to 14 percent, 10 percent, 9 percent, and then 8 percent, before finally moving to zero after the UTTC has been fully implemented. This is somewhat slower migration than a straight application of price elasticity would indicate.