A goal of introductory physics courses is to improve students' conceptual understanding of forces and assist them to obtain a Newtonian framework on which they can build further understanding. In both algebra-and calculus-based introductory physics sequences, students completed the Force Concept Inventory (FCI) at the beginning and end of the first course (mechanics) and then again at the beginning of the second course (electricity & magnetism, optics). Since some students delayed taking the

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... ond course, a wide range of time gaps exists between the two courses with no intervening instruction in physics. We use a database containing six years of assessments to investigate how retention of conceptual understanding depends on time. We also examine whether the completeness of the Newtonian framework affects retention of force conceptual understanding. I. edited by Ding, Traxler, and Cao; Peer-reviewed,