The purpose of this paper is to argue that the measurement of child outcomes is the major stumbling block in estimating the extent to which equality of opportunity exists for children in our nation's schools. This paper has three parts. The introduction discusses the concept of equal educational opportunity, as defined and then redefined over the past five years. The second part identifies the main focus of this paper, that members of the methodological community have ignored basic measurement problems and have directed their efforts towards analysis and design issues and to debates about what to measure, when assessing outcomes of schooling. Consideration of both is necessary to achieve educational reform and accountability. The third part describes the critical problems in measurement that must be resolved before effectiveness of schools can be assessed: when to measure, how to measure, how to interpret the size of an effect, and how to gauge the extent of program implementation when interpreting the results of social experiments about school effects. (Author/JH)