The puzzling popularity of the PPO.

Surging growth in preferred provider organization (PPO) participation has been fueled by migration away from the undesirable features of health maintenance organizations (HMOs). While employers, consumers, and providers seem to know what it is they do not want from HMOs, the advantages offered by PPO design are not so clear. This is attributable in part to difficulties in determining what a PPO arrangement actually is. But it may also reflect a lack of strong evidence that PPOs control costs, provide active care management, or promote quality improvement.