Selection bias in health maintenance organizations: analysis of recent evidence.

An analysis of recent research regarding selection bias in health maintenance organizations (HMO's) is presented in this article. Review of the available literature leads one to conclude that prepaid group practice HMO's do experience favorable selection. It has been demonstrated in numerous studies that prior use of health services by HMO enrollees is less than prior use of health services by those who remain in the fee-for-service sector, and there is considerable evidence that shows a statistically significant positive relationship between prior use and current use. This is true for both those under 65 years of age and those 65 years of age or over.