This paper presents an econometric model to summarize these reimbursement incentives, present information on cost sharing and generosity in the PPS model, and discuss the implications of these findings. McClellan points out repeatedly that the PPS has increasingly permitted more retrospective adjustments to the payments. This has been primarily driven through outliers and treatment-driven DRG's developed during the admission, rather than diagnosis-driven DRG's identified at the time of admission. Outlier payments allow for supplemental payments for unusually lengthy or expensive admissions and make up no more than 5% of all hospital admissions. The author considers the increase in case mix index, a measure of the intensity of care delivered, to have the most important influence on overall PPS payments. He adds that these retrospective adjustments may be hampering the program's effectiveness in implementing cost sharing. McClellan's model allows individual aspects of PPS to be evaluated for their contribution to cost sharing rates and reimbursement variance. He used linear regression to approximate how the cost sharing reimbursement is derived, using fixed and variable components. Lower costs equaled lower reimbursement; however, higher costs led to higher reimbursements only if the costs were driven by a procedure. This was most true for men aged 65 to 69 with surgery requiring ICU care and an increased length of stay. Generosity estimates indicate that virtually all hospitals do some cost sharing. Higher generosity is usually associated with greater retrospective limits on cost sharing. In the 1990s, high-tech hospitals saw more generous reimbursements and were more likely to survive, whereas for-profit hospitals were more likely to exit the market.