To develop and test a model for both describing and predicting Iowa school districts' pupil transportation costs, the author first tested the generalizability of a 1980 study by McAvoy of 37 Iowa districts and then identified independent variables that would explain Iowa pupil transportation costs. Data were gathered on all 447 Iowa districts for the 1978-79 school year. Statistical analysis allowed the 1980 study to be replicated using the statewide data; the results indicated that the earlier study's findings--that essential elements in a state transportation aid formula should be the total number of pupils transported and the total number of miles traveled--could be generalized to the entire state. Further analysis identified a third independent variable, the total number of buses in a district's fleet. Testing of 367 mathematical manipulations of these three variables revealed one standard unit of measure that correlated most highly with total pupil transportation costs. The standard unit consists of the square root of the sum of two variables (miles and buses), multiplied by the square root of the number of pupils. A linear regression equation based on this unit explains 93 percent of the variance in pupil transportation costs. (Author/RW)