From the late 1960s to the early 1980s, there was a surge of research on both Keller's and Bloom's instruction methods. Most of these studies showed that mastery learning has a positive effect on achievement, for all subjects and at all levels. Also, mastery learning brings positive affective outcomes for both students and teachers. These studies also showed that there are many variables that are either affected by mastery learning or that influence it somehow: student entry variables, curriculum, type of test, pacing, level of mastery, and time.