Crude percentages are calculated as the number of people with a particular characteristic in a population under study, divided by the number of people in that population, multiplied by 100. In this report, crude percentages based on denominators less than 100 people were not considered reliable and not presented (marked n.p).
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. Age-standardised proportions 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 under study.
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 0–17, and 18–34 up to age 75 and over were used to calculate the age-standardised percentages.
Throughout this report, the age-standardised percentage is referred to as ‘prevalence’.
Age-specific percentages are useful for comparing the results across age groups when results are strongly age-dependent. The age-specific proportions are calculated by dividing the number of events occurring in each specified age group in a population of interest by the total number of people in the same age group of that corresponding population, multiplied by 100.
The comparison of age-specific results can be cumbersome when it requires many comparisons. Additionally, breaking the data further by the age groups, can lead to many small populations that have insufficient size to maintain the confidentiality and the reliability of the data. In order to maximise the number of populations to report, the age-specific percentages were calculated, using the broad age groups, 0–44, 45–64 and 65 and over.
Analyses were conducted through the ABS DataLab, which is a secure environment, allowing virtual access to files for the users to undertake real time analysis of data, including the 2021 Census, in MADIP. Analytical outputs are required to be vetted by the ABS before being cleared outside the DataLab. In this report, some data are supressed and not presented (marked n.p.) based on the requirements of the ABS to manage confidentiality.
For more information on the ABS DataLab, and its ‘Output rules’, see DataLab.