Bloom has looked to mastery learning, exemplified by the Personalized System of Instruction (PSI), to reduce individual differences in posttest performance, such as final grades and midterm examination scores. While PSI courses have raised overall performance, there has been little examination of individual differences in these courses. Data from an undergraduate PSI course in educational psychology were analyzed for the relation of errors on unit quizzes to midterm examinations and final performance. The midterms and finals were constructed of the same questions used in unit quizzes, but not all questions had been seen by all students. Errors on unit quizzes were most closely related to midterm and final exams. Bloom's model was only supported when single units or the number of attempts at mastery were studied. The model was not supported when total errors leading to mastery were used. Several scatter plots showed that a critical percentage of unit quiz errors led to sharply increasing dropoffs of midterm scores. Questions which were new to students were more affected by total error level. Bloom's model applies best to students who make few errors in achieving mastery. More effort needs to be directed to the students who make many errors. (Author/MH)