Assessing the relationship between volume and outcome in hospital services: implications for service centralization.

Proposals for centralizing services are often justified on the basis of studies linking the volume of activity to the outcomes achieved. However, the evidence of such studies is far from demonstrating a causal link between volume and outcome. This article assesses the main reasons why volume and outcome studies do not in themselves demonstrate a causal link, and therefore do not provide adequate support for proposals for centralizing hospital services. It then sets out a number of precepts to guide those responsible for proposing centralization of services.