Little known is that since the No Child Left Behind Act (NCLB) was passed, the federal government has prioritized "school finance equity" as a goal for states to achieve. In fiscal year 2008, 21 percent of NCLB Title I funds will be distributed based on an Education Finance Incentive Grant formula, a funding stream that has been increasing since NCLB first passed. But few people understand why certain states are deemed more "equitable" than others. You hear a lot of praise and criticism about equitable school funding, but little explanation of what it means or what produced it.
School finance equity typically measures how much per-pupil expenditures vary across districts within a given state. Greater equity means less variation. Sounds simple, but there are many different formulas used to calculate it, and their specifics can be quite complicated (remember statistics and weighted coefficients of variation?). The federal government has its own definition of school finance equity, which you can read more about here from www.EdBudgetProject.org.
Some states are more equitable than others for two basic reasons: (1) decisions made at the state level about how to distribute funding, and (2) the size and number of school districts within a state.