Age-standardisation is a method of reducing the influence of age when comparing two or more populations with different age structures. In this report, the direct age-standardisation approach was used. Direct age standardisation applies the age-specific results to a 'standard population' in order to determine the proportion or rate that would have occurred if the populations under study had the same age distribution as the 'standard population'. The method provides the age-standard proportions or rates as single summary measures. The 2001 Australian Standard Population was used to calculate age-standardised proportions and age-standardised rates.
Results based on small populations, or a small number of events are unreliable and exhibit a large amount of random variation and are therefore not presented in this report. Age-standardised rates were not presented (marked n.p.), if the total number of events was less than 20 over all age groups or the denominator was less than 30 in any one age group, for any population.
Direct standardisation may not remove all confounding and produce precise age adjustment if the categories used are not sufficiently narrow. Conversely, age groups that are too fine can introduce excessive volatility into the age-standardised rates. Having no events in an age group can produce misleading results, since cells with zero events are assumed to have zero variance, resulting in an underestimation of the true variance. For many populations of interest, the number of events was zero at some specific ages, particularly younger ages, regardless of the health outcome. 10-year age groupings starting with the age group 0–9, and up to age 80 and over were used to calculate the age-standardised rates in this report.