FEATURE ARTICLE 2: METHODOLOGY USED TO PRODUCE PRELIMINARY ABORIGINAL AND TORRES STRAIT ISLANDER POPULATION ESTIMATES FOR 30 JUNE 2016


INTRODUCTION

The preliminary estimate of the Aboriginal and Torres Strait Islander population at 30 June 2016 was 798,400 people. This estimate comprises of 649,200 people counted on Census night, plus the 138,000 people measured as net undercount and the inclusion of an additional adjustment of 11,200 people. This article describes the method used to compile the preliminary 2016 rebased estimate of the Aboriginal and Torres Strait Islander population.

The preliminary Aboriginal and Torres Strait Islander population estimate is based on the 2016 Census of Population and Housing and is calculated using a similar process to that used for the total Australian population. The diagram below shows an overview of the steps taken. For further information see, Australian Demographic Statistics December Quarter 2016 (cat. no. 3101.0) Feature Article: Rebasing Australia's Population Estimates Using the 2016 Census.

In calculating the Aboriginal and Torres Strait Islander population, additional steps, including smoothing of net undercount estimates and demographic treatment for age heaping for the Northern Territory, were made to ensure a robust estimate was produced.


THE 2016 CENSUS OF POPULATION AND HOUSING

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 were a result of imputation by the ABS where a dwelling (either private or non-private) was deemed to be occupied and people were 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

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.

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).

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.


NET UNDERCOUNT

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

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 the preliminary 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.

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).

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.

Comparison between PES and Empirical Bayes Standard Errors - 30 June 2016
	PES standard error	Empirical Bayes standard error
	%	%
New South Wales	3.5	2.1
Victoria	10.1	4.5
Queensland	4.5	2.6
South Australia	7.5	4.1
Western Australia	6.0	3.4
Tasmania	5.7	4.2
Northern Territory	2.1	1.7
Australian Capital Territory	12.6	7.3
Australia	2.1	2.1

CALCULATING REBASED ESTIMATED RESIDENT POPULATION

Estimates of the Aboriginal and Torres Strait Islander population on Census night, which have been adjusted for net undercount (as noted above) were then further adjusted to compile population estimates at 30 June. This involved adding back Australians temporarily overseas at the time of the Census, and backdating to the 30 June using data on births, deaths and interstate and overseas migration, which occurred between the two dates. Minor demographic adjustments were then made to address anomalies in age and sex composition. All these additional adjustments added in 11,200 people at the national level.


Treatment for Age Heaping in the Northern Territory

When respondents are unsure of their age or the age of others they are reporting on behalf of, they may round their ages to those ending in 0 or 5. This produces systematic spikes for ages ending in 0 or 5, a phenomenon known as age heaping.

Age heaping is a relatively common demographic issue in developing countries and is generally not observed in the total Australian population, except in the Northern Territory, where it is evident in Census counts and population estimates for the Aboriginal and Torres Strait Islander population.

While the ABS has not traditionally applied an adjustment for age heaping, the increasing demand for data by single year of age for key reporting purposes has necessitated a demographic treatment to ensure the best possible estimates are produced by age for the Northern Territory’s Aboriginal and Torres Strait Islander population.

The preliminary estimates of the Aboriginal and Torres Strait Islander population for the Northern Territory released in this publication are adjusted for age heaping using the Sprague method. For further information, please see Technical Note 2 in Estimates of Aboriginal and Torres Strait Islander Australians, June 2011 (cat. no. 3238.0.55.001).

Components of the Aboriginal and Torres Strait Islander Population - 30 June 2016
	Census Count	Empirical Bayes Undercount (plus)(a)	PES Estimate	Additional Adjustments (plus)(b)	Estimated Resident Population
New South Wales	216 200	45 200	261 400	4 200	265 600
Victoria	47 800	9 000	56 800	1 000	57 800
Queensland	186 500	32 200	218 600	2 800	221 400
South Australia	34 200	7 600	41 800	500	42 300
Western Australia	76 000	22 700	98 700	1 800	100 500
Tasmania	23 600	4 700	28 300	300	28 500
Northern Territory	58 200	15 500	73 700	800	74 500
Australian Capital Territory	6 500	900	7 400	100	7 500
Australia(c)	649 200	138 000	787 200	11 200	798 400
(a) Includes undercount of the Aboriginal and Torres Strait Islander population for those Census records for which Indigenous status was unknown.
(b) Incorporates residents temporarily overseas and backdating components.
(c) Includes Other Territories.

PLANS FOR FURTHER OUTPUT

Final resident population estimates by Indigenous status will be available in August 2018 in the publication Estimates of Aboriginal and Torres Strait Islander Australians (cat. no. 3238.0.55.001). For more information regarding estimates of Aboriginal and Torres Strait Islander population please e-mail demography@abs.gov.au.