Public school funding in Massachusetts is based on foundation budget principles. However, funding formula modifications often create disparities between district foundation budgets and actual required spending levels. This study provides an in-depth look at Massachusetts' state aid formulas used between 2004 and 2009 and utilizes two approaches to measure the effects of aid modifications on vertical and horizontal equity. The first is a regression-based approach which compares the intended effects of student characteristics on foundation spending to the actual effects on required spending. The second approach measures equity directly for each school district and uses the Tobit estimation technique to examine the effects of student and district characteristics on the equity measure. Results indicate that district characteristics such as community wealth and regional school systems increase the likelihood of a district benefiting from aid modifications. Holding community characteristics constant, there is also evidence that districts with high proportions of low income elementary school students and/or English language learners benefit from overlay provisions as well.