To determine whether individual differences in student achievement and learning rate are reduced or eliminated by mastery instruction, 166 Navy trainees who had completed a computer-managed course in basic electricity and electronics were cluster-analyzed into groups, using 24 measures of cognitive characteristics. Discriminant analyses were computed between the two derived groups using module test scores and completion times. Groups differed significantly in their achievement in 4 out of 11 modules and in the time required to complete 1 module, but did not demonstrate a progressive decrease in the variability of their achievement and learning rates. Twenty-eight references are listed. (Author/LMM)