Evaluating Medicaid HMOs when encounter data are missing: case of developmentally delayed children

In evaluating Medicaid Health Maintenance Organizations (HMOs), crucial information regarding severity of illness of patients is often missing--in part because encounter data are not available. If we assume that patients are either in the HMO or in fee-for-service (FFS) plans (i.e., no in or out migration); then severity of HMO patients can be deduced from encounters of FFS patients. We applied this approach to effectiveness of HMO services for developmentally delayed children. Data supported the assumption of a closed system. Data also showed that over 12 months, severity of FFS patients declined. Therefore, we inferred that the HMO was attracting sicker patients. The HMO was paid less than FFS plan, despite the fact that it attracted sicker patients.