An in-depth investigation of the relationship between media effectiveness, essentiality, and amount of study was presented. Overall, effectiveness explains 48% of the variation in the amount of study. Students who found the media more effective studied the media more. Those media that were more effective were studied more. The relationship between effectiveness and amount of study is different for each medium. This is because media differ in their degree of "essentiality-versus-optionality." Thus, the amount of study depends on a combination of effectiveness and essentiality. The relationship between effectiveness, essentiality, and amount of study is different for each student. This is because students differ in their "sensitivities"--their sensitivity to effectiveness and to essentiality. The final equation used effectiveness, essentiality, and sensitivity to explain 74% of the variation in the amount of study. The analysis was informed by a mathematical model.