Many US hospital-affiliated freestanding ambulatory surgery centers are located on hospital campuses, relevant to interpretation of studies involving ambulatory surgery

The percentage of freestanding surgery centers located within 250 yards of main hospital buildings is sufficiently large to influence analyses. When using US national data, ambulatory surgery reported as performed at a hospital should not be considered as having been performed within the hospital. Similarly, hospital affiliated freestanding surgery centers should not be assumed to be more than a 5 min walk for anesthesia and operating room personnel from the hospital.