Understanding the Social Risk Factor Adjustment’s Effect on Star Ratings


CMS implemented the Categorical Adjustment Index (CAI) to address measurement bias in the Medicare Advantage (MA) Star Ratings, as unadjusted scores may disadvantage MA contracts serving more enrollees at greater social risk. CAI values are added to a contract’s Star Ratings to adjust for the mean within-contract performance disparity associated with its percentage of enrollees with low socioeconomic status (ie, receipt of a Part D low-income subsidy or dual eligibility for Medicare and Medicaid [LIS/DE]) and who are disabled. We examined the CAI’s effect on Star Ratings and the type of contracts affected.

Study Design

Observational study of MA contracts with health and prescription drug coverage.


We compared adjusted and unadjusted 2017-2020 Star Ratings overall and by contracts’ proportion of LIS/DE and disabled enrollees. We assessed the CAI’s effect on qualifying for quality bonus payments (QBPs), eligibility for rebate payments, and high-performing and low-performing designations.


The CAI’s impact was modest overall (3.2%-14.9% of contracts experienced one-half Star Rating changes). Upward changes were concentrated among contracts with high percentages of LIS/DE or disabled enrollees (7.7%-32.3% of these contracts saw increased Star Ratings). In 2020, 26.0% of contracts with a high proportion of LIS/DE or disabled enrollees that qualified for a QBP did so because of the CAI.


The CAI primarily affected contracts with high LIS/DE or disabled enrollment, which received higher Star Ratings because of the CAI. The adjustment helps ensure that such contracts’ performance is not understated and reduces incentives for MA contracts to avoid patients at greater social risk.

Research conducted by

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