Age adjustment

In epidemiology and demography, age adjustment, also called age standardisation, 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) for the general Australian population, and specifically for the Indigenous Australian population. In each age category over age 24, Indigenous Australians had markedly higher rates of circulatory disease than for 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+.

However, overall, these surveys estimated that 12% of all Indigenous Australians had long-term circulatory problems compared to 18% of the overall Australian population.

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 is still higher than for non-Indigenous peers of the same age.

In order to get a more informative comparison between the two populations, a weighting approach is being used. Older groups in the Indigenous population are weighted more heavily (to match their importance 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, representing the fact that Indigenous Australians do have a higher risk of circulatory disease.

To adjust for age under this direct method of standardization, age-specific rates in each group must be known, as well as the age structure in a standard population.

Age adjustment is commonly used when comparing prevalences in different populations; it is also used for characteristics such as life expectancy, average income, and other properties that are not directly linked to the total population size.

Age adjustment is 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).

See also

• Age-standardized mortality rate
• Simpson's paradox

Further reading

• 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.