 The difference between weak and strong students would then be that the stronger ones be able to finish the course quicker, while the weaker ones would need more time.The Keller Plan has mainly been used in higher education, particularly as a more personalized form of instruction in large classes, but there is nothing inherent in Keller's formulation to restrict its application to particular grade levels, content, or types of courses; for instance the papers  and  report on usage in elementary school and junior high school, respectively.There has been a good deal of research on the effectiveness of PSI which indicated that it had robust, significantly positive effects on learning when compared to more traditional lecture-based formats.