Diagnosis-related group (DRG) is a system to classify hospital cases into one of originally 467 groups. The system of classification was developed as a collaborative project by Robert B Fetter, PhD, of the Yale School of Management, and John D Thompson, MPH, of the Yale School of Public Health. The system is also referred to as "the DRGs," and its intent was to identify the "products" that a hospital provides. One example of a "product" is an appendectomy. The system was developed in anticipation of convincing Congress to use it for reimbursement, to replace "cost based" reimbursement that had been used up to that point. DRGs are assigned by a "grouper" program based on ICD (International Classification of Diseases) diagnoses, procedures, age, sex, discharge status, and the presence of complications or comorbidities. DRGs have been used in the US since 1982 to determine how much Medicare pays the hospital for each "product," since patients within each category are clinically similar and are expected to use the same level of hospital resources.