Book excerpt: Evidence of patient and physician turnover in accountable care organizations (ACOs) has raised concerns that ACOs may be earning shared-savings bonuses by selecting for lower-risk patients or providers with lower-risk panels. We conducted three sets of analyses to examine risk selection in the Medicare Shared Savings Program. First, we estimated overall MSSP savings through 2015 using a difference-in-differences approach and methods that eliminated selection bias from ACO program exit or changes in the practices or physicians included in ACO contracts. We then checked for residual risk selection at the patient level. Second, we re-estimated savings with methods that address undetected risk selection but could introduce bias from other sources. These included patient fixed effects, baseline assignment, and area-level MSSP exposure to hold patient populations constant. Third, we tested for changes in provider composition or provider billing that may have contributed to bonuses, even if they were eliminated as sources of bias in the evaluation analyses. We find that MSSP participation was associated with modest and increasing annual gross savings in the 2012-2013 entry cohorts of ACOs that reached $139-302/patient by 2015. Savings in the 2014 entry cohort were small and not statistically significant. Robustness checks revealed no evidence of residual risk selection. Alternative methods to address risk selection produced consistent results but were less robust than our primary analysis, suggesting the introduction of bias from within-patient changes in time-varying characteristics. We find no evidence of ACO manipulation of provider composition or billing to inflate savings. We further demonstrate that exit of high-risk patients or physicians with high-risk patients from ACOs is misleading without considering a counterfactual among non-ACO practices. We conclude that participation in the original MSSP program was associated with modest savings and not with favorable risk selection. These findings suggest an opportunity to build on early progress. Understanding the effect of new incentives and opportunities for risk selection in the revamped MSSP will be important for guiding future program reforms.