The Australian Census Longitudinal Dataset (ACLD) was used to determine the factors for the identification change assumption series, which were then applied to the medium demographic assumptions. The 2016-2021 ACLD is a representative 5 per cent sample of individuals linked across the 2016 and 2021 Censuses and includes records for 31,894 Aboriginal and Torres Strait Islander people (people whose Indigenous status was Aboriginal and/or Torres Strait Islander in 2016 and/or 2021). The ACLD contains responses to the Census Indigenous status question, so changes in identification can be quantified. The 2016-2021 ACLD sample was used for this analysis, as this is the most recent linked file and has the largest sample size.
Identification change ratios used in the identification change assumption are calculated by dividing the number of people who identified as being of Aboriginal and/or Torres Strait Islander origin on the 2021 Census by the number of people who identified as being of Aboriginal and/or Torres Strait Islander origin on the 2016 Census. The identification change ratios are calculated by age and geography.
Age groups are treated as a cohort, meaning that people aged 15 years in the 2021 Census and people aged 10 years in the 2016 Census are used when calculating the ratios. Identification change ratios for single year of age showed considerable variation, due to small sample counts in some groupings of geography by age. Therefore, age was combined into four categories (0-14 years, 15-24 years, 25-49 years, and 50 years and over) based on similar patterns of identification change within these categories.
Identification change ratios are calculated for three levels of geography: national, state/territory, and Remoteness Areas. Identification change ratios were not calculated for Indigenous Regions, due to low ACLD sample sizes in some regions.
There was not much difference in identification change between males and females, so combined identification change ratios were calculated, rather than separate ratios for each sex.
For the 2016-2021 ACLD sample (weighted to give population totals), identification change ratios by age category (0-14 years, 15-24 years, 25-49 years, and 50 years and over, as reported in the 2016 Census) and geography (national, state/territory, and Remoteness Areas, as reported for the place of usual residence in the 2021 Census) are calculated as:
\(\large{Ratio_{age_{2016},geography_{2021}} = \frac{nACLD2021_{age_{2016},geography_{2021}}}{nACLD2016_{ age_{2016},geography_{2021}}}}\)
This five-year identification change ratio is converted to an annual factor by taking the fifth root and applied as an additional assumption only in the identification change projection series.
\(\large{AnnualFactor_{ age_{2016},geography_{2021}} = \sqrt[5]{Ratio _{ age_{2016},geography_{2021}}}}\)
Identification change ratios are calculated using ACLD sample weights, however identification change ratios calculated without weighting were also analysed and produced very similar results.