A group of students normally distributed in aptitude and given the same instruction will produce a normal distribution of student achievement. It has been contended that if each of five primary variables in learning are optimized for each student, all students should be expected to achieve mastery of the material. These variables are (1) aptitude of student, (2) quality of instruction, (3) ability to understand instruction, (4) perserverance, and (5) time allowed for learning. This study investigated this hypothesis with an individualized learning program (ILP), in which all students were supposed to attain mastery on each lesson before going on in the program. Students in grades two through six were given aptitude tests, and their performance in the ILP was compared with the test results. Little relationship between rate of learning and aptitude was found when variables number two, three, and four were ignored or were assumed to be operating at an optimum level for all. Thus, either the variables should not have been ignored or the experimental design in this study was faulty. Aptitude may still be found to be the most important factor in rate of learning. (WD)