Assessing the evidence on HMO performance.

HMOs contract to provide physician and hospital services to enrolled populations in return for fixed periodic payments. Total costs for HMO members are shown to be lower than for those enrolled in conventional insurance plans. But the explanation for this and related differences cannot be attributed solely to matters of efficiency, quality, or even consumer self-selection or physician satisfaction. Although public policy assumes that HMOs will encourage beneficial competitive responses by traditional health care providers, too little is known to predict the nature and extent of such probabilities.