To the Editor: In their study of patient case-mix adjustment, hospital process performance rankings, and eligibility for financial incentives, Dr Mehta and colleagues1 tackled an important question in health services research. However, the unit of analysis they used in their study seems inconsistent. For example, in the Methods section, the authors stated that the analysis used opportunity-based data. Later, however, the authors stated that they adjusted for opportunity mix in the analyses. It seems that adjustment should be performed at the patient level, not the opportunity level. Furthermore, the authors stated that the hospital rankings were based on the scores for composite process measures. This again indicates the unit of analysis should be patient, not opportunity (ie, quality measure), because each patient has a composite score summarizing 8 quality measures.2
Tsai C. Analyzing Patient Case Mix and Hospital Rankings. JAMA. 2009;301(11):1125. doi:10.1001/jama.2009.304
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