Directly Standardised Rate (DSR). Mortality rates, hospital admission rates, incidence rates and other rates can be calculated for different geographical areas, but can also be applied to other measures such as percentages.

For example, a mortality rate would be calculated as the number of deaths out of the total population, and could be expressed as the number of deaths per 100,000 population, and thus the deaths divided by the population figure is then multiplied by 100,000. 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 total populations of the same size, the geographical area with the older population would tend to have a higher mortality rate. The mortality rate would be higher simply because that geographical area had an older population. If we wish to compare two areas to see their underlying mortality rates are different, then we need to adjust for differences in the age and gender structure of their populations. 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. Indirect standardisation produces Indirectly Standardised Ratios or a Standardised Mortality Ratio (SMR) which is a ratio of two rates.

Direct standardisation produces a standardised rate or a Directly Standardised Rate (DSR). The DSR involves applying the rates of condition (deaths, admissions, new cases of the disease etc.) observed in the study or local population to a ‘standard’ population. For example, when calculating a directly standardised mortality rate this would involve calculating the number of deaths that would have occurred in the standard population if the age-specific (mortality) rates of the local population were applied to the standard population. The DSR is generally standardised to the European Standard Population (the most recent version is the ESP 2013) which is an artificial fixed population with a set number of people in each age group. When the DSR is referred to within this document, it is generally referring to a mortality rate, although directly standardised incidence rates are presented in relation to cancer. All mortality DSRs within this document are standardised to the ESP 2013, and are given as the number of deaths per 100,000 population (essentially the number of deaths that would have occurred within the ESP per 100,000 population). Once the DSR has been produced for a specific time period, it is not necessary to recalculate it when new data is available (for the next period of time) as the population is fixed, although this is not the case when using indirect standardisation (as direct standardisation uses a standard population at a point in time, e.g. comparing Hull’s mortality rate for 2017-19 with England’s mortality rate for 2017-19 as the standard population, and when new data for 2018-20 is available, the mortality ratios are generally recalculated with the data for England for 2018-20 as the new ‘standard’ population).

In most cases within our JSNA direct standardisation has been used to calculate a rate, but it is also possible for the method of direct standardisation to be used to produce another statistical measures such as a percentage. Percentage standardised or adjusted for age and gender have been used in the analysis of data (percentages) from our local Health and Wellbeing Surveys, particularly the Young People Health and Wellbeing Survey where percentages by group (such as local deprivation fifth) have been adjusted for the age and gender structure so any differences in the percentages are then attributable to the group (e.g. deprivation) rather than differences in the age or gender structures between the groups.

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.

Indirect standardisation involves calculating the age-specific (mortality or hospital admission) rates in the standard population and applying them to the local population for each age group. This gives an expected number of ‘events’ for each age group, and these expected number of events are summed over all age groups. The standardised ratio is the expected number of events divided by the observed number of events (summed over all age groups). The value is then generally multiplied by 100. An standardised ratio of 100 means that the event rate in the population of interest is the same as in the standard population after adjusting for differences in the age structures of the two populations. A standardised ratio of more than 100 denotes a higher event rate in the population of interest compared to the standard population, and a ratio of less than 100 denotes a lower even rate.

Also see Crude Rate, European Standard Population and Indirectly Standardised Ratios and Standardised Mortality Ratio.