A mix of cases in a hospital reflects the diversity, clinical complexity and the needs for resources in the population of patients in a hospital. Hospital patients are classified into groups based on main and secondary diagnoses, procedures, age, complexity (comorbidities) and needs. These groups are known as Diagnosis Related Groups (DRGs). Each DRG has a specific measurement that is related to the amount of resources needed to treat that condition in that specific kind of patient. The different mix of all the DRGs in a hospital is called the Case mix index.

In the U.S., Case Mix Index (CMI) is the average DRG weight for a hospital’s Medicare patient population. It can be used to adjust the average cost per patient for a given hospital relative to the adjusted average cost for other hospitals by dividing the average cost per patient by the hospital’s calculated CMI. The adjusted average cost per patient would reflect the charges reported for the types of cases treated in that year. If a hospital has a CMI greater than 1.00, their adjusted cost per patient or per day will be lowered and conversely if a hospital has a CMI less than 1.00, their adjusted cost will be higher.