BACKGROUND: The Consumer Assessment of Health Plans Study (CAHPS) health plan survey is currently administered to large independent samples of Medicaid beneficiaries and commercial enrollees for managed care organizations that serve both populations. There is interest in reducing survey administration costs and sample size requirements by sampling these 2 groups together for health plan comparisons. Plan managers may also be interested in understanding variability within plans. OBJECTIVE: The objective of this study was to assess whether the within plan correlation of CAHPS scores for the 2 populations are sufficiently large to warrant inferences about one from the other, reducing the total sample sizes needed. RESEARCH DESIGN: This study consisted of an observational cross-sectional study. SUBJECTS: Subjects were 3939 Medicaid beneficiaries and 3027 commercial enrollees in 6 New Jersey managed care plans serving both populations. MEASURES: Outcomes are 4 global ratings and 6 report composites from the CAHPS 1.0 survey. RESULTS: Medicaid beneficiaries reported poorer care than commercial beneficiaries for 6 composites, but none of the 4 global ratings. Controlling for these main effects, variability between commercial enrollees and Medicaid beneficiaries within plans exceeded variability by plans for commercial enrollees for 4 of the 10 measures (2 composites, 2 global ratings). CONCLUSIONS: Within-plan variability in evaluations of care by Medicaid and commercial health plan member evaluations is too great to permit meaningful inference about plan performance for one population from the other for many important outcomes; separate surveys should still be fielded.