An important feature of recent large-scale performance assessments has been the reporting of pupil and school performance in terms of performance or proficiency categories. When an assessment uses such ordered categories as the primary means of reporting results, the natural way of reporting on the quality of the assessment is through the probabilities of consistent and correct classification of students. This paper applies a method introduced by S. Livingston and C. Lewis (1995) for calculating those probabilities. The use of this procedure to extend F. Lord's strong true score theory to tests containing other than multiple-choice items has created an important tool for test developers. The data used in the paper are from the New Standards Reference Examinations in Mathematics and English-Language Arts that were administered to students in Grades 4, 8, and 10 in spring 1996. The results of these analyses show that for total score composites, the range of students accurately classified as having "met the standard" is from 85% to 98%, and the range of students consistently classified as having "met the standard" is from 77% to 96% across grades and content areas. (Contains 4 tables and 13 references.) (Author)