In epidemiology and demography, age adjustment, also called age standardization, is a technique used to better allow populations to be compared when the age profiles of the populations are quite different.
For example, in 2004/5, two Australian health surveys investigated rates of long-term circulatory system health problems (e.g. heart disease) in the general Australian population, and specifically in the Indigenous Australian population. In each age category over age 24, Indigenous Australians had markedly higher rates of circulatory disease than the general population: 5% vs 2% in age group 25–34, 12% vs 4% in age group 35–44, 22% vs 14% in age group 45–54, and 42% vs 33% in age group 55+.
To understand this apparent contradiction, note that the Indigenous population is comparatively young (median age 21 years, compared to 37 for non-Indigenous) due to relatively high birth and death rates. Because of this, Indigenous figures are dominated by the younger age groups, which have lower rates of circulatory disease; this masks the fact that their risk at each age is higher than for non-Indigenous peers of the same age.
To get a more informative comparison between the two populations, a weighting approach is used. Older groups in the Indigenous population are weighted more heavily (to match their prevalence in the "reference population", i.e. the overall Australian population) and younger groups less heavily. This gives an "age-adjusted" morbidity rate approximately 30% higher than that for the general population, indicating that Indigenous Australians have a higher risk of circulatory disease. (Note that some residual distortion remains due to the wide age bands being used.) This is directly analogous to the standardized mortality ratio for mortality statistics.
To adjust for age under this direct method of standardization, age-specific rates in each group must be calculated, as well as the age structure of the standard population.
When is it used
Age adjustment is commonly used when comparing prevalences in different populations. It is not used to derive life expectancy, which is calculated directly from the age-specific mortality rates, with no reference population required.
Age adjustment is also not appropriate when attempting to compare population totals (for instance, if we wanted to know the total number of hospital beds required for patients with circulatory diseases).
- "National Aboriginal and Torres Strait Islander Health Survey". Australian Bureau of Statistics. 2006. Retrieved 2009-01-12.
- "National Health Survey: Summary of results". Australian Bureau of Statistics. 2006. Retrieved 2009-01-12.
- "4704.0 — The Health and Welfare of Australia's Aboriginal and Torres Strait Islander Peoples, 2008". Australian Bureau of Statistics. 2008. Retrieved 2009-01-12.
- Lee WC, Liaw YP (October 1999). "Optimal weighting systems for direct age-adjustment of vital rates". Stat Med 18 (19): 2645–54. doi:10.1002/(SICI)1097-0258(19991015)18:19<2645::AID-SIM184>3.0.CO;2-Q. PMID 10495462.