Liao JM, Gupta A, Zhao Y, et al. - In value-based payment models, medicare has used both voluntary and mandatory participation to engage health care organizations. Voluntary participants are assumed to achieve greater savings because they self-select into programs due to greater opportunity to reduce spending compared with mandatory participants. For the Care Improvement program, Medicare started the voluntary Bundled Payments in 2013, which included hip and knee joint replacements. In 2016, for the Joint Replacement program, under the Comprehensive Care, Medicare randomized hospitals in 75 metropolitan statistical areas to receive mandatory hip and knee joint replacement bundled payments, whereas hospitals in 121 metropolitan statistical areas continued receiving fee-for-service payments. This produced an opportunity to analyze the relationship between voluntary vs mandatory hospital participation in a bundled payment program for joint replacements and episodic spending changes.