1. Average relative weight of all cases treated at a facility or by a certain physician that reveals the clinical severity of a defined group in relation to other groups in the classification system. Formula: Divide the sum of the weights of diagnosis-related groups (DRGs) for patient discharged during a specified period by the total number of patients discharged. A low CMI may indicate DRG assignments that do not adequately reflect the resources used to treat Medicare patients. 2. In prospective payment systems, this is the comparison of a hospital’s cost for its case mix to the national or regional average hospital cost for a similar case mix.