The goal of mastery learning is to have all students reach a prescribed level of mastery (i.e. 80–90% on a test). In order to achieve this, some students will require more time than others, either in practice or instruction, to achieve success. The Time-Achievement Equality Dilemma refers to this relationship between time and achievement in the context of individual differences. If you hold achievement constant, time will need to vary. If time is held constant (as with modern learning models), achievement will vary. According to its critics, mastery theory doesn't accurately address this relationship.