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TECHNICAL NOTE 1 ESTIMATED RESIDENT ABORIGINAL AND TORRES STRAIT ISLANDER POPULATION – METHOD OF CALCULATION
2 The Australian Bureau of Statistics (ABS) used the results of the 2016 Census of Population and Housing as the main data source to produce the rebased population estimates for Aboriginal and Torres Strait Islander Australians. The Census counted 649,200 Aboriginal and Torres Strait Islander people in Australia (including Other Territories) on Census night. This excludes 1,411,000 Census records (6.0% of the total Census count) with Indigenous status unknown. Of these, 14% or (0.9% of the total Census count) were a result of item non-response; that is, the ABS received a partially completed Census form for the person, with the Indigenous status question unanswered.
The remaining and majority (86%) of records with unknown Indigenous status (5.2% of the total Census count) for which no Census form was received and the dwelling (either private or non-private) was deemed to be occupied, then had people imputed into it. While some of the records will be for people of Aboriginal and/or Torres Strait Islander origin and others for non-Indigenous people, no imputation was made for Indigenous status in Census data. These Census counts were then adjusted using data collected in the Census Post Enumeration Survey.
THE CENSUS POST ENUMERATION SURVEY
3 The Census Post Enumeration Survey (PES) is a household survey conducted by the ABS shortly after each Census, that provides an independent measure of Census coverage. The 2016 PES included people from approximately 42,000 fully responding households across Australia. Information was collected for everyone present in the household. In addition to obtaining basic demographic information, questions were asked about each person's usual residence, their location on Census night, and any other addresses where they might have been counted in the Census. The PES results are used to determine how many people should have been counted in the Census, how many people were missed, how many were counted more than once, and how many were counted in error. Net undercount is defined as the difference between the PES estimate of the number of people who should have been counted in the Census and the actual Census count.
4 Net undercount is used as one of the inputs for compiling the Aboriginal and Torres Strait Islander population estimates. In the PES, Indigenous status is collected from every person (i.e. there is no non-response). The PES is therefore able to estimate the undercount of the Aboriginal and Torres Strait Islander population for those Census records for which Indigenous status was not stated (i.e. the undercount of the Aboriginal and Torres Strait Islander population in the Census due to non-response).
5 To estimate net undercount for the Aboriginal and Torres Strait Islander population, the PES imputed unknown Indigenous status where it was unknown in the Census. Specifically, a not stated Indigenous status was imputed as either Aboriginal and Torres Strait Islander or non-Indigenous, according to the distribution of stated responses within each age group, sex, Census form type and geographical area (Statistical Area Level 2) according to PES.
6 In the 2016 Census, the net undercount of the Aboriginal and Torres Strait Islander population was 138,000 people. This equates to a net undercount rate of 17.5%, which is slightly higher than the rate for the 2011 Census (17.2%). This undercount number is derived by calculating the difference between the PES Aboriginal and Torres Strait Islander population estimate (787,200) and the Aboriginal and Torres Strait Islander population Census count (649,200).
EMPIRICAL BAYES ESTIMATION
7 Some state and territory estimates of undercount for the Aboriginal and Torres Strait Islander population have relatively high standard errors and therefore the raw PES estimates are not used in calculating Aboriginal and Torres Strait Islander population estimates below the national level. Instead, the Empirical Bayes method (as used in 2011) is used to produce smoothed undercount rates for 18 regions of Australia. Each of these regions was a customised geographic area designed to capture the varied collection issues in different parts of Australia, as an input into the Empirical Bayesian estimation method. Victoria, Tasmania, South Australia and the Northern Territory were separated into two regions (split between capital city and balance of state), while New South Wales, Queensland and Western Australia were separated into three regions (with the balance of state split). The Australian Capital Territory was treated as a single region.
8 This method smooths the raw PES estimate of the undercount based on the Census characteristics of the region (specifically the proportion of Aboriginal and Torres Strait Islander persons and the level of Census non-response in the region).
9 The aim of smoothing is to provide stable estimates with lower standard errors. The overall amount of smoothing used in each region was determined by two factors; the standard error for each region and an overall smoothing constant. The smoothing constant was chosen using a ‘method of moments’ technique developed by Morris (1983). Regions with high standard errors required more smoothing. The outcome of this methodology, in relation to smaller standard errors and confidence intervals is presented in the table and the graph below.
These additional adjustments added in 11,200 people at the national level.
11 The table below shows preliminary and final estimates as well as 2011 Census-based projections of the Aboriginal and Torres Strait Islander population, for 30 June 2016. For Australia, the final estimate remained largely unchanged from the preliminary estimate published on 27 September 2017 in Australian Demographic Statistics, March 2017 (cat. no. 3101.0).
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