Service-level selection by HMOs in Medicare.

In the federal Medicare program, contracting health maintenance organizations (HMOs) are paid on a capitated basis. There has long been concern that an "adverse selection" of risks remain in the traditional fee-for-service (FFS) sector, since beneficiaries with low costs may leave the FFS sector and join the HMOs. The distortion associated with this form of selection is that health plans may design their mix of health care services in order to effectuate favorable selection. This paper scrutinizes patterns of HMO membership and costs by service in the FFS sector for evidence consistent with the hypothesis that HMOs engage in service-level product distortion. We develop a multi-service model of choice between FFS and HMOs and show that if the HMO sector is underproviding (overproviding) a service relative to the FFS sector, we should observe a positive (negative) correlation between the HMO market share and average costs of those remaining in the FFS sector. We estimate the correlation between the HMO market share and the average FFS costs for different health care services using Medicare data for 1996. We find evidence indicating that there exists significant service-level selection by HMOs.