Responses to the Consumer Assessments of Health Plans Survey (CAHPS) are related to respondent characteristics. CAHPS procedures include casemix adjustment to remove effects of difference in respondent characteristics on comparative plan ratings, under the assumption that casemix coefficients are homogeneous across plans. The authors analyzed data from the Washington state CAHPS demonstration, fitting hierarchical models in which coefficients of casemix variables and intercepts could vary by plan. They estimated the impact of variability in casemix coefficients on plan adjustments and also assessed the implications for differential effects of individual characteristics at different plans. Estimated between-plan variability of coefficients was small, but the data are consistent with substantially larger variability. The potential impact of this variability on adjustments for plans was small relative to the magnitude of the adjustments. Comparisons between plans for individuals, however, could be affected substantially. This methodology could be useful wherever casemix adjustment is applied.