A crude rate is generally a count of the occurrence of an event (such as a hospital admission or death) divided by the population, and often as the rates are relatively small, the crude rate is expressed as the number of events per 10,000 population or per 100,000 population. There is a strong association between age and hospital admission, and between age and death so in many cases, it could be misleading to compare the crude hospital admission rates or crude mortality rates between two geographical areas if the population structure differs significantly. In general, a geographical area with an older population will tend to have a higher rate of hospital admissions and a higher mortality rate associated with its population. However, many other factors are associated with increased likelihood of hospital admissions and death, such as deprivation. In many cases, when two (or more) geographical areas are compared, one wants to assess if there is a difference after adjusting for or taking into account differences in the age structure of the population. This can be done by standardisation and there are two different methods of standardisation: indirect and direct standardisation.
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.
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 directly standardised rate is multiplied by 1,000 or 100,000 for ease of presentation, and the standard population used for direct standardisation is generally the European Standard Population.
In some cases, rates are presented as crude rates, and this is generally acceptable if the age is limited to a specific narrower age range. Most of the information presented on The Office for Health Improvement & Disparities’ Fingertips and on our JSNA website that is associated with hospital admissions relating to 0-18s are given as crude rates, as there is less of a difference in the age structures within children and young people, although the hospital admission rates for alcohol-specific conditions and substance misuse among young people has been standardised (as there will be more admissions among the older age groups).
Also see Directly Standardised Rate and Indirectly Standardised Ratios and Standardised Mortality Ratio.