A term used in the prospective payment system (PPS). It is a measure of the “expected costliness,” per patient, of treating a given hospital’s mix of cases. CMI is scaled so that a hospital whose mix is like that in base data would have a CMI of 1.0. The base data for a CMI usually come from a broad sector of inpatients, often the Medicare patients, discharged from the nation’s hospitals during a base year.