Standardised mortality measures, such as the hospital standardised mortality ratio (HSMR) or the summary hospital mortality indicator (SHMI), are presented as a ratio of actual to expected mortality. An expected mortality rate is calculated for each hospital using data derived from discharge coding: using a statistical model to forecast the number of deaths that a hospital would be expected to have, based on the characteristics of the admitted patients. Because expected mortality rates are based on discharge coding, it is important for clinicians to support accurate coding (for example, by avoiding the use of symptom diagnoses such as ‘chest pain’). The SHMI and HSMR have a number of important differences:  

SHMI  HSMR  
Data source  Discharge codes  Discharge codes 
Methodology  Statistical model to forecast expected number of deaths  Statistical model to forecast expected number of deaths 
Mortality measure  Inpatient deaths or death within 30 days of discharge  Inpatient deaths 
Included patients  All deaths  Exclusions for certain diagnoses 
Standardised measures are presented using statistical process control (SPC) methodology with 100 as the reference value and upper and lower control limits calculated (analogous to confidence intervals). Control limits are usually set so that the probability of a value lying outside them by chance is less than 2 per 1,000. Rates are reported as abnormal if they are outside the control limits. Although HSMR and SHMI are often presented as a single monthly or quarterly figure, this is unhelpful without knowledge of the control limits and the pattern over time. Observing changes in mortality data over time is generally a much more useful guide to quality than a oneoff measurement.  
Common problems with hospital SMRs:  

Adapted and reproduced from RCP’s Acute care toolkit 11.^{15}