How Benchmark Changes Affect Participation in Accountable Care Organizations: Prospects for Voluntary Payment Models.

In voluntary alternative payment models, participation is essential for model viability and the progression of provider payment reform. We study participation decisions of accountable care organizations (ACOs) in the Medicare Shared Savings Program (MSSP). We leverage a natural experiment in which a 2017 rule change introduced differential shocks to an ACO's baseline spending and performance-year spending. The net effect was an effective change in benchmarks that varied across ACOs. Dropout was 7 percentage points more likely among ACOs with effective benchmark decreases. While small reductions in the effective benchmark did not affect program participation, larger reductions increased dropout by 11 percentage points. ACOs with spending already above their benchmarks were particularly sensitive to effective benchmark reductions, consistent with the program's weak long-term incentives to reduce spending. Our results highlight the causal role benchmarks play in determining ACO participation and the need to consider the consequences of participation effects in the design of new payment models.