Evaluating and improving the measurement of hospital case mix.

The foundation of case-based prospective payment is the case classification system. The purpose of classification systems is to group together patients with similar treatment requirements. The systems described in this issue take a variety of theoretical and practical approaches to classification. The critical issue in comparing these systems is whether the variation in treatment requirements which is not explained by the classification system is associated with particular groups of patients, particular hospitals, or particular groups of hospitals in such a way as to result in unfair reimbursement. We suggest criteria for comparing classification systems and a research agenda for clarifying the fairness of different approaches.