Disease and mortality rates, as well as other health and welfare indicators, are often used to evaluate the performance of government and community programs aimed at improving the health and welfare of the population. This involves comparing event rates for different populations over time or examining the trends in a particular population over time.
Populations whose event rates (e.g. death, disease, hospital separation or birth rates) are being compared may often have different age structures. A given population may also have different age distributions over time as a result of changes in death rates, fertility rates and migration. Because these event rates vary by age, comparisons are often confounded by differences in the age distributions of the populations being compared. For example, comparison of indicators of wellbeing between the Aboriginal and Torres Strait Islander population and non-Indigenous Australians is hampered by the different age distributions of the two populations.
Age-standardisation is a technique used to enhance the comparability of event rates from different populations or different sub-populations over time by making adjustments for the confounding effects of differences in age structure between the populations being compared.
There are two main methods of age-standardisation, the direct and indirect methods. Whether to use the direct or indirect method depends on the purpose of the analysis, what type of comparison is being carried out, including whether trend analysis is being carried out, as well as on a number of data quality considerations. Among the data quality considerations are the following:
- the availability of accurate and reliable event rates, classified by age, in the study populations
- the overall number of events in each of the study populations
- the distribution of events by age in the study populations
- the size of the overall population at risk and in each age group
- the number of populated cells
- the consistency of the relationship between age and the event of interest in each of the study populations.
After exploring these issues, this report recommends that the direct method of age-standardisation be used for purposes of comparing health and welfare outcome measures (e.g. mortality rates, life expectancy, hospital separation rates, disease incidence rates etc.) of the Aboriginal and Torres Strait Islander population and non-Indigenous Australians. In particular, the direct method of age-standardisation should be used to compare the gap in wellbeing between Aboriginal and Torres Strait Islander and non-Indigenous Australians. The direct method of age-standardisation is also recommended for use in investigating changes in health and welfare outcome measures for the Aboriginal and Torres Strait Islander population over time.
The report provides guidance on when and how to use the direct age-standardisation method and under what circumstances it should not be used.