Measuring and Comparing Mortality
8.39 The Standardised Mortality Ratio (SMR) is a method of comparing mortality levels in different years, or for different sub-populations in the same year, while taking account of differences in population structure.
8.40 An SMR is calculated by comparing an observed number of deaths to an expected number, multiplied conventionally by 100. Thus, if mortality levels are higher in the population being studied than would be expected the SMR will be greater than 100. Similarly, the Standardised Registration Ratio (SRR) is calculated in the same way but using the number of disease registrations rather than mortality.
8.41 The Compendium of Clinical Indicators provides the majority of the data in this section. The compendium uses the population of England as a reference and therefore the SMR for England is always 100. Consequently, if an SMR is below 100 the study population has lower rates of mortality than England as a whole.
8.42 Confidence Intervals (CIs) for SSRs and SMRs provide information regarding the precision of estimated values since they take into account the variability of the SMR estimate. Throughout this section two numbers in brackets following an SMR, e.g. 93 (92-94), denote the lower and upper values of a 95% confidence interval for the SMR estimate. These lower and upper values define a range of values within which we can be 95% sure the real value for the SMR lies. An SMR is significantly high if both lower and upper values of the CI are above 100, and low if both are below 100.