Overlap in HMO physician networks.

Health maintenance organizations' (HMOs') restrictions on the size of their physician networks may facilitate cost containment and quality improvement activities but may also impede access to care and impose barriers to those wishing to switch health plans or jobs. We examine the extent, variation, and predictors of overlap in HMO physician networks. We predict that people who switch HMOs have a reasonable likelihood (50 percent) of being able to retain their physician. Overlap ranges from an upper quartile of 69 percent to a lower quartile of 34 percent. Group/staff-model HMOs have little overlap, while younger plans, for-profit plans, and plans in small markets have greater overlap.