For 10 years or more, teachers at the City Colleges of Chicago have used mastery learning, a teaching technique that focuses on the use of a corrective/feedback process to improve student learning. Early studies of mastery learning at the colleges comparing student grades in mastery learning classes to control classes in which these techniques were not used demonstrated a positive effect, with significantly more students earning credit grades in mastery classes than in comparable non-mastery classes. A recent study sought to determine whether differences in the amount of exposure to mastery learning methods could be related to performance. The colleges' computer-based student recordkeeping system provided information on students' grades and the instructional method used in their courses. A comparison of the earned credit ratios (ECR's) of students taking no mastery learning classes, one mastery learning class, and two mastery learning classes revealed: (1) in eight of nine comparisons, mastery learning sections had higher ECR's than comparable non-mastery learning sections; (2) students who took no mastery learning classes had an overall ECR of 64%, while the ECR of those taking one mastery learning class was 59% and that of students taking two mastery learning classes was 66%; and (3) there were no significant differences in the average difficulty of the non-mastery learning classes taken by mastery learning students and other students. (LAL)