Addition of fiscal response factors to the traditional simulation model of school finance demonstrates that the traditional model underestimates both the costs and impacts of state aid programs at the district level. The study reviews research on fiscal response--defined as school districts' taxing and spending behavior after receipt of state aid--and extends the fiscal response analysis both by measuring fiscal response over two-year (instead of one-year) periods, using three different statistical methods; and by adding to the fiscal response equations the effects of geographic cost differences, local noneducational spending, and school district wealth and income. Combining the augmented fiscal response equations with the traditional simulation model of school finance allows one to predict individual school districts' responses to state aid programs. Applied to New York State data, the combined model shows that low-income districts spend a greater proportion of state aid on education than high-income districts, and that the traditional model underestimates district spending increases in response to state aid as well as the impact of state aid on equity goals and the costs of New York's "percentage equalizing" method of financial aid. (Author/RW)