8.9 Age standardisation

Population phenomena are dependent on age. Comparisons of mortality in different populations, for instance, are often complicated by differences in age structures. A useful tool for purposes of comparing populations with different age structures is provided by age standardisation. Comparisons of populations that have different age structures are based on the so-called standard population. In a study of mortality, for example, age standardisation is based on age-specific mortality rates that are used to "transfer" the mortality of the study populations into the standard population.

There are two methods of age standardisation. In the direct method both the age-specific mortality rates and the age structure of the study population are known, in the indirect method only the total number of deaths in the population and its age structure are known.

Direct age standardisation:

Direct age standardisation uses the age-specific mortality rates for the study population to compute the number of people who would die in the standard population. In this method the mortality rates for the study population are applied directly to the standard population and the crude mortality rate is calculated.

Indirect age standardisation:

Indirect age standardisation uses the age-specific mortality rates for the standard population to compute the number of people who would die in the study population. The next step is to compare the number of expected and observed deaths. This gives the indirectly standardised mortality ratio (SMR).