The fourth report in a series on school finance describes how state school finance formulas are differentiated and the calculations used to measure local need, wealth, and effort. The following topics are addressed: (1) criteria for evaluating state school finance programs; (2) origins of state school aid formulas; (3) operation of equalization formulas; (4) variables affecting state aid programs; and (5) impact of court-ordered reform on school finance. Charts explain how minimum foundation programs and the formulas for calculating percentage equalization aid work and how guaranteed tax bases and guaranteed tax yields affect formula outcomes. The conclusion of this report suggests that any proposal for changing funding formulas must consider the dynamics of the state-school district relationship that may enhance or limit the equalizing of fiscal opportunities and fiscal outcomes. (JAM)