Essential to the definition of an effective school is an operational definition of minimum academic mastery as the acquisition of knowledge, skills, and concepts that enable the student to be demonstrably successful in the next grade level in the same school or in any other school in the United States. An effective schools program, such as the one developed in the Spencerport (New York) school system, implements this achievement component and demonstrates the following criteria: (1) at least 95 percent of all students at each grade level demonstrate minimum academic mastery (measured on a standardized achievement test); (2) there is no significant difference in the proportion of students demonstrating minimum academic mastery as a function of socioeconomic class; and (3) the above conditions have been obtained for at least three consecutive years. To determine whether a school is effective, a disaggregated analysis of student achievement in reading and arithmetic must be computed at each grade level. The procedure for computing a disaggregated analysis (derived from a series of inservice workshops with Lawrence Lezotte and Ronald Edmonds) concludes this article. A postscript, describing the Spencerport project, and four footnotes are appended. (IW)