The primary studies forming the basis of these reviews come largely from the United States, where for-profit and not-for-profit providers work side by side in the same environment, and where large administrative databases allow accurate detection of death rates. But, while these observational studies are threatened by bias from patient selection – sicker patients mean higher death rates, for example ­– the available administrative databases document patient characteristics such as age, severity of illness, and co-morbidity, including diabetes and high blood pressure.