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.