This clinical classification system is proprietary and is published and maintained by 3M. It was created to better represent resource use in all inpatient settings, not just the Medicare population. APR-DGRs add four subclasses to each DRG (see encyclopedic entry: Diagnosis Related Group) regarding severity of illness and risk of mortality. Patient age is also evaluated in this classification system, which is used by all payers as a predictive model for resource use and outcomes.