Mastery Learning has been one of the most highly investigated teaching methods over the past 50 years. While it has been the subject of high criticism, it has also been found to have resounding success when implemented correctly. A meta-analysis by Guskey & Pigott (1988) looked at 46 studies that implemented group-based mastery learning classrooms. Results found consistently positive effects for a number of variables including "student achievement, retention of learned material, involvement in learning activities, and student affect". However, a notable variation was found within student achievement and it was believed this was due mainly to the subject being taught. Courses such as science, probability, and social studies yielded the most consistent positive results, while other subjects were varied.