Investments by physicians in specialty hospitals, ambulatory surgery centers, and other competing facilities have caused many full-service acute care hospitals a great deal of concern. These full-service hospitals believe they are at a disadvantage in competing against such centers because physician-investors allegedly can "pre-select" profitable patients. Full-service hospitals have reacted in a variety of ways, including adopting economic credentialing policies. Physician-owned centers often view these policies as anti-competitive and seek relief under the antitrust laws. Hospitals attempt to defend these actions on the grounds that they prevent what economists call "free-riding." A key element of this defense is whether pre-selection is occurring and whether the hospital has sufficient evidence of pre-selection to justify the conduct under the antitrust laws.