Appendix B The Census, Post Enumeration Survey, and the Estimated Resident Population

Report on the quality of 2021 Census data: Statistical Independent Assurance Panel to the Australian Statistician

An independent view of the quality of statistical outputs from the 2021 Census of Population and Housing



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The ABS produces a range of collections related to population statistics: the Census, the Census Post Enumeration Survey, and the Estimated Resident Population.

It is important to note that the following description of these collections is drawn from pre-existing ABS documents and the Report on the Quality of 2016 Census data. Therefore, the text that follows is either taken directly from those documents or informed by them.

B.0.1 The Census of Population and Housing

The ABS conducts a Census of Population and Housing every five years as required by the Census and Statistics Act 1905.[1] Regularly taking a census provides a comprehensive snapshot of the nation and enables the updating and maintenance of an accurate time series of Australia’s official population estimates.

The Census counts everyone based on where they were on Census night (place of enumeration) and asks about their place of usual residence. The Census also collects data on a broad range of personal, family, and dwelling topics including Ancestry, Country of birth, Income, Language used at home, Marital status, Family size, Occupation, and Dwelling type.


[1] Census and Statistics Act: Census and Statistics Act 1905 (Cth)

B.0.2 Post Enumeration Survey

No matter how much care is taken in the preparation of the Census, there will be some people who are not counted (undercount) or those that are counted more than once (overcount). For the reasons explained below, it is important to quantify the undercount and overcount. For countries without population registers, the recommended method is to conduct a Post Enumeration Survey following as soon as possible after the Census[1]. The ABS has been conducting a Post Enumeration Survey since 1966.

The Post Enumeration Survey is a sample survey of approximately 50,000 dwellings, conducted soon after the completion of the Census enumeration period. The Post Enumeration Survey checks if a person should have been counted on Census night, by asking for their Census night location. The Post Enumeration Survey also determines if they were counted; and if so, how many times they were counted. This is done by directly linking Post Enumeration Survey persons and dwellings to their matching Census forms. The difference provides the ABS with a net overcount or net undercount, which is then applied as part of the five yearly Estimated Resident Population rebasing exercise (discussed at Section B.2, below). A diagrammatic presentation is provided below in Figure B.1.

The Post Enumeration Survey therefore provides:

  • a critical component to Census rebasing of the Estimated Resident Population;
  • an independent measure of the accuracy and coverage of the Census counts; and
  • assistance in identifying improvements for future censuses.


[1]   See United Nations (2017). ‘Census Recommendation to support demographic analysis’, in Principles and Recommendations for Population and Housing Censuses, Revision 3.  Available at

Figure B.1 Estimating the population using the Census and the Post Enumeration Survey

A diagram illustrating how the Census and the Post Enumeration Survey are used to provide estimates of Australia's population as described in the paragraphs above and the description for this image.
This image is a diagram of how the population is estimated using the Census and the Post Enumeration Survey. The first main box is: Census finds most but not all persons in the Australian population. A side box feeds into the main box: Undercount is the persons missed in the Census. The second main box is: The Post Enumeration Survey mostly finds the same people that Census finds, but…. There are three side boxes feeding into this second main box: 1. The Post Enumeration Survey finds some people that the Census didn’t; 2. The Census finds some people that the Post Enumeration Survey didn’t; and 3. Some are not found by the Post Enumeration Survey or by the Census. The third main box is: It’s okay for the Census and the Post Enumeration Survey not to find everyone, as long as the chance is independent. Three side boxes feed into this third main box: Suppose the Census doesn’t find 3 per cent…; And suppose the Post Enumeration Survey doesn’t find 4 per cent…; … then we estimate that 3 per cent of 4 per cent (or 0.12 per cent) were not found by either the Census or the Post Enumeration Survey.

B.0.3 Estimated Resident Population

The official estimate of Australia’s population is the Estimated Resident Population (often referred to as ERP). It provides the number of usual residents in a geographic area, by age and sex. In the Census, a person is counted as a usual resident if they usually live in Australia or if they usually live in another country but are resident in Australia for one year or more. In the context of overseas migration, a person is determined to be a usual resident if they spend at least 12 out of 16 months in Australia.

Estimated Resident Population statistics are used to inform evidence-based decisions such as allocating funds to the states and territories and allocating the number of seats for the House of Representatives between the states and territories. More recently, the Estimated Resident Population has been used as the denominator in calculating COVID-19 vaccination rates.

The Estimated Resident Population at the national and state/territory level is prepared quarterly and is based on data from the latest Census (adjusted for the Post Enumeration Survey), to which components of natural increase and decrease (births and deaths) and migration (internal and overseas) are added. Data for these components are from government administrative sources

B.1 How the Post Enumeration Survey is used to assess the quality of the Census

B.2 How the Census and Post Enumeration Survey combine to produce Estimated Resident Population

B.3 Limitations of the Post Enumeration Survey

B.4 Key changes to the 2021 Post Enumeration Survey

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