A meta-analysis of forty-nine comparative studies showed that mastery testing generally has positive effects on student learning, but the size of effect in any single study depends on both the stringency of the mastery criterion used with the mastery group and the degree of experimental control employed in the study. Mastery effects were also more pronounced on the less able students in a classroom and were less noticeable in more able students. Finally, the analysis showed that mastery testing increased the amount of time needed for instruction.