- Alex Bottle, lecturer in medical statistics,
- Brian Jarman, emeritus professor,
- Paul Aylin, clinical reader in epidemiology and public health
- 1Dr Foster Unit, Department of Primary Care and Public Health, Imperial College London, London EC1A 9LA
- Correspondence to: A Bottle
- Accepted 10 September 2010
Hospital standardised mortality ratios (HSMRs) are intended as an overall measure of deaths in hospital, a proportion of which will be preventable. High ratios may thus suggest potential problems with quality of care. Although a growing number of countries are using HSMRs, they are controversial, especially if the figures are made public, as in England and Canada.1 2 3
The HSMR is complex but cheap and relatively easy to calculate from national or other benchmark data that allow calculation of patients’ predicted risks of death. However, there are a number of methodological challenges in their construction and interpretation, which we discuss below. Although there are other versions of the HSMR, we focus on the Jarman one.4 Full methodological details of its construction in England are given on bmj.com. A few of the finer points that we discuss are specific to English hospital data, but most of the methodological concerns are relevant to HSMRs (or other composite hospital mortality measures) in any developed country.
What is an HSMR?
The HSMR is derived from administrative data commonly used for billing purposes from hospital information systems such as Hospital Episode Statistics in England. It is the ratio of the observed to expected deaths, multiplied by 100, with expected deaths derived from statistical models that adjust for available case mix factors such as age and comorbidity.
The HSMR is meant as an overall measure of adjusted in-hospital mortality and serves as a screening tool. Some of the deaths in the numerator will be preventable. Thus some of the variation in HSMRs between hospitals will be due to important variation in preventable deaths, although much will be …