In a previous study1 the writer developed a student's ability index based upon the age progress of the student through school combined with the measure of his success in meeting the curriculum requirements as indicated by the teachers' marks. This index was determined by the formula : Mk MOA SAI =-X CA GMk in which SAI indicates any student's ability index, Mk is the student's average mark in the grade in which the work is done, CA is the student's chronological age, MGA is the mental grade-age, that is, the average mental age of students working in the same grade, and GMk is the grade mark or the average mark obtained by students in the grade. The prac tical use of the formula depended upon finding a proper value for the MGA second member, -, as a constant. To this end the value of MGA GMk was taken from the well-known work of Terman and others on 1,936 California children. For GMk several series of SAI's were computed for 508 students in School B of a town system, assuming varying values of GMk in each series.2 It was to be desired that the distribution of the