Although the topic of effective schools is a popular one among today's educators, researchers, and publishers, there still is no commonly agreed upon definition of an effective school. One definition given by Ronald Edmonds and Lawrence Lezotte is notably precise, measurable, attainable, and easy to determine. They defined an effective school as one which demonstrates the following criteria: (1) 95 (or greater) percent of all students at each grade level demonstrate minimum academic mastery and are prepared to succeed in the next grade anywhere in the United States; (2) there shall be no significant difference in the proportion of students demonstrating minimum academic mastery as a function of socioeconomic class; and (3) the above two conditions have been obtained for a minimum of three consecutive years. Minimum academic mastery is measured by performance on a standardized test (preferably criterion-referenced as opposed to norm-referenced). A disaggregated analysis can be used to determine school effectiveness as well as to establish school district standards higher than those set by a state education department. It also is a simple, effective tool for proving or disproving hypotheses or questions. Eleven footnotes are appended. (IW)