Later in the 1960s Benjamin Bloom and his graduate students were researching individual differences in school learning. They observed that teachers displayed very little variation in their instructional practices and yet, there was a lot of variation in student's achievements. Bloom used Carroll's conceptual model to create his own working model of Mastery Learning. Bloom realized that, if aptitudes were predictive of the rate at which a student can learn (and not necessarily the level to which), it should be possible to fix the degree of learning expected to some mastery level and then systematically manipulate the variables in Carroll's model such that all or almost all students attained that level.