Health service planners are increasingly trying to find ways to improve the quality and safety of health care. A wide range of approaches is being used from high level regulatory frameworks, use of clinical guidance and guidelines, to more micro level activity such as audit of care. None of these is easy; all require significant investment of resources, training, time and monitoring. The results are often uneven and result in variations in quality as initiatives diffuse unevenly through the system. It is understandable then that policy makers seek easier ways to deliver these improvements.

Research since the late 1970s seemed to point in the direction of a relatively constant relationship in health care—increased use of a hospital procedure reduces the mortality associated with it. The message emerging from a large number of studies, mainly from the USA, was that patients treated in hospitals which (or by clinicians who) managed high volumes of patients with the same condition had better outcomes than those with lower volumes. This was summed up by Luft and others in an influential report in 1990.¹ Hundreds of studies have been published, many of which are based on analysis of large US administrative databases, and most report an inverse relationship between the volume of activity and mortality or other poor outcomes. These studies have been reviewed and summarised in several publications in the last few years.^2–5 Although there was not full agreement, in general the reviews—especially those published in the USA—support the hypothesis of a volume-outcome relationship and the existence of volume-quality thresholds.

The policy response was predictable. Several national and state regulators and professional associations set volume thresholds for hospital based procedures and for hospital accreditation and pushed for the …