Standardised Mortality Ratio (SMR), standardised incidence ratio or standardised hospital admission ratio. Mortality rates, hospital admission rates, cancer incidence rates and other rates can be calculated for different geographical areas. Producing a standardised ratio is a way of calculating the rate adjusting for the age structure of the populations, so that any differences found are due to differences in the rate due to some other factor like deprivation (rather than simply a difference due to the age structure of the population). The ratio is a rate compared to another rate (generally the rate for England). The most common standardised ratio relates to mortality so the focus of the text below relates to mortality, but any rate can be standardised and presented as a ratio.
For example, a mortality rate would be calculated as the number of deaths out of the total population. Often it is useful to compare the resulting rates with other geographical areas to ascertain whether the rate is particularly high or low in relation to the national rate (e.g. the rate for England) or in relation to similar geographical areas (e.g. in terms of deprivation). However, when rates from two or more different geographical areas are compared, any differences among the rates could be due to differences in the age and gender structure of the populations. For two areas with a total population the same size, the geographical area with the older population would tend to have a higher mortality rate. It is possible to ‘standardise’ the rate using a standard population to ‘adjust’ the resulting rate so differences in the age (and gender) population structures are taken into account. Direct standardisation produces a Directly Standardised Rate (DSR). Indirect standardisation produces a standardised ratio or in the case of mortality a Standardised Mortality Ratio (SMR).
The SMR involves applying the rates of disease in a ‘standard’ population to the study or local population. For example, when calculating an SMR, this would involve using the age-specific (mortality) rates within the standard population and applying them to the local population. In this document, the SMR is generally standardised to the English population, so the age-specific mortality rates for England are applied to the age and gender structure of the local population to calculate the expected number of deaths in the local population. This expected number of deaths is then divided by the observed number of deaths within the local population to produce a ratio, and it is often multiplied by 100 so an SMR of 100 denotes that the mortality rate of the local population is the same as the standard population after taking into account the differences in the age and gender structure. In most cases, a SMR relates to mortality, but the same methodology can be used to report on indirectly standardised hospital admission rates or standardised incidence rates (for cancer). However, within this website, an SMR generally relates to mortality, with England as the standard population. If the SMR is more than 100 then the local mortality rate is higher than England, and can be expressed as the relative differences compared to England. For instance, an SMR of 130 denotes that the mortality rate of the local area is 30% higher than England (after adjusting for the population structure), and an SMR of 89 denotes that the local mortality rate is 11% lower than England (after adjusting for the population structure). As the age-specific mortality rates change over time in the standard population, it is necessary to take this into consideration when examining trends over time. All SMRs give the (mortality) position relative to England’s mortality rates at a specific point in time. Thus if an SMR has been produced for a specific geographical area for 2017-19 (i.e. in relation to England’s age-specific mortality rates), then when new data is available (for the next period of time), it is necessary to recalculate the SMR, and the SMR for 2018-20 could also be recalculated for the geographical area which show the area’s mortality relative to England 2018-20. In general, the trends over time will be based on the SMR using the most recent age-specific mortality rates in the standard population. This means that the SMR for the standard population will be 100 for the latest period, but will different for earlier years. For all causes combined, mortality rates have been decreasing, so the SMRs for England will generally be higher for earlier years. The SMR can increase even when mortality rates have reduced in the local area as they reflect the relative mortality position relative to England. For instance, if the age-specific mortality rates have all fallen in the local area, but the SMR has increased over the same time period, then this would simply reflect the fact that the mortality rates have fallen in the local area but at a slower rate compared to England thus inequalities have increased.
Crude rates are not adjusted and simply represent the number of events (hospital admissions or deaths, etc) divided by the population, and are often presented as the number of events per 1,000 population or 100,000 population. Crude rates have been presented in the JSNA in relation to A&E attendances and hospital admissions among children and young people where the age range is narrower, so it is not necessary to present the standardised rate.
Direct standardisation produces a directly standardised rate and applies the uses the age-specific (mortality or hospital admission) rates of the geographical area or population of interest and applies them to the standard population to estimate the number of ‘events’ in the standard population. The total number of expected events over all ages is then divided by the total population in the standard population (summed over all ages) to give the standardised rate. In most cases the standard population used for direct standardisation is the European Standard Population.
Also see Crude Rate and Directly Standardised Rate.