Cooperative mastery learning (Guskey, 1990; Mevarech, 1991) combines mastery learning with cooperative learning (discussed in chapter 15). A teacher using cooperative mastery learning could introduce skills and concepts to students who are grouped heterogeneously, and then the students could discuss and practice the skills and concepts in cooperative groups. At the end of each unit, the students could be tested individually; and those who have not attained mastery could correct their learning in the small groups. During this process, the teacher could move from group to group and provide assistance as needed. This combination of strategies is a good example of attacking Bloom's (1984) two-sigma problem by integrating principles that focus on different aspects of learning.