This paper presents findings of a study that sought to determine whether a composite score is a more appropriate dependent variable in the measurement of school effectiveness than any one component score. The viability of using a composite score, such as the School Effectiveness Index (SEI), was compared with the viability of using any one of five different subject areas. Specifically, the study examined the level of consistency that exists when school-effectiveness-classification models are held constant across test scores. Multiregression analysis of data from 315 Louisiana high schools was used to predict student achievement on a statewide-administered criterion-referenced test (CRT) from indices of socioeconomic status (SES), racial makeup, school report card variables, and demographic data. Findings indicate that a composite index incorporates the varied information that each component issue provides and presents a better overall picture of school effectiveness than would any one test component. In conclusion, a school's success in one subject does not necessarily ensure success in other areas. The composite score is preferable for school-effectiveness research, because it provides more information and is more reliable than a subtest. Two tables and one figure are included. (LMI)