The Personalized System of Instruction: A Flexible and Effective Approach to Mastery LearningFox, E. J. (2004). The Personalized System of Instruction: A flexible and effective approach to mastery learning. In D. J. Moran & R. W. Malott (Eds.), Evidence-based educational methods (pp. 201-221). San Diego: Elsevier Academic Press. This chapter provides an overview of the Personalized System of Instruction (PSI), a flexible and mastery-based model of teaching developed by Fred Keller and others in the 1960s. The key features of the model are defined and redefined, and the empirical support for the approach is reviewed. It is argued that PSI remains a compelling and attractive model for course development.