We evaluate the efficacy of a blended formative/summative assessment tool developed to support mastery learning by students without placing undue burden on instructors. Our innovation provides students with an opportunity to take a more difficult “challenge” quiz to demonstrate their command of the material and improve their grade on regular in-class quizzes. The structure of these quizzes motivates students to modify study behaviors (formative component) and take responsibility for knowledge acquisition (summative component). This mastery-based testing approach serves to bring the student's objective of a quality grade in line with the instructor's objective of quality learning.