An experiment was conducted to determine the effects of simultaneous variation of experimental variables and individual difference variables. The experimental variables were response effortfulness, stages of practice, and distribution of practice; the individual difference groups were college students and mental defectives. An attempt was made to predict the results on the basis of response effortfulness. Although the interpretation of results predicted on the basis of response effortfulness was not clear due to unexpected behavior of the effort variable, it was found that the patterns of performance were the same in both individual difference groups.