Standardized mortality ratios.

Mortality rates are increasingly used to compare the performance of hospitals and of units within hospitals. Different approaches have been taken to develop 'standardized' measures which adjust for the 'expected' mortality based on key characteristics of the population being treated. This article discusses the original motivation for looking at hospitals in this way in the UK, describes two recent incidents in which such measures were a critical element in decision making and reviews some of the strengths and weaknesses of the approaches. The current practice on the use of standardized mortality ratios is described.