Adjusting performance measures to ensure equitable plan comparisons.

When comparing health plans on scores from the Medicare Managed Care Consumer Assessment of Health Plans (MMC-CAHPS) survey, the results should be adjusted for patient characteristics, not under the control of health plans, that might affect survey results. We developed an adjustment model that uses self-reported measures of health status, age, education, and whether someone helped the respondent with the questionnaire. The associations of health and education with survey responses differed by HCFA administrative region. Consequently, we recommend that the case-mix model include regional interactions. Analyses of the impact of adjustment show that the adjustments were usually small but not negligible.