Medicare Shared Savings Program (MSSP) accountable care organizations (ACOs) must annually choose between prospective and retrospective patient assignment. This decision affects patient assignment and ACO operations, benchmark, expenditures, and shared savings. For agreement periods starting January 1, 2024, or later, regional expenditures are calculated under prospective or retrospective attribution based on the ACO’s selected alignment method (prospective or retrospective), rather than defaulting to retrospective. This brief explores how assignment methodology affects key ACO metrics under current MSSP rules.

Key questions addressed:

• How does assignment choice affect key ACO metrics? Metrics such as expenditures per beneficiary per year (PBPY), risk scores, risk-adjusted expenditures PBPY, and regional efficiency are all influenced by attribution method.
• What is the difference between prospective and retrospective assignment? We provide a primer on the choice, discussing assignment windows and advantages of methodology, differences in assigned populations, and financial implications.
• How should an ACO decide which method to use? The best assignment methodology will be ACO-specific and should be evaluated in light of baseline and performance years, as well as operational differences between the methodologies.