Methods and quality assurance of preliminary rebased estimated resident population (ERP) at 30 June 2021 and ERP revision back to 30 September 2016
Rebasing is the revision of the estimated resident population (ERP) based on the latest Census.
Australia's ERP includes all people who usually live in Australia (regardless of nationality, citizenship or visa status).
Between Censuses, we calculate quarterly ERP by adding the flows of population change (births, deaths and migration) to the stock population estimate (the base).
The ABS undertakes a Census of Population and Housing (the Census) every five years as required by the Census and Statistics Act 1905.
The Census helps to create a base population estimate, which we update each quarter for the following five years in ERP, until the next Census when we repeat the process.
A regular Census:
The difference between this new rebased ERP and the quarterly component-based estimate (known as unrebased ERP) is called ‘intercensal difference’.
To ensure a consistent time series, we revise quarterly ERP between the current and previous Censuses.
The preliminary rebased population estimates are available in National, state and territory population, December 2021.
Final population estimates will be available in June 2023, following final revisions to components in the rebasing process.
We calculate the preliminary rebased ERP at 30 June in a Census year by:
This new preliminary rebased ERP becomes the base for quarterly estimates going forward from that point until the next Census.
The steps are illustrated in the following diagram.
| NSW | Vic. | Qld | SA | WA | Tas. | NT | ACT | Aust.(a) | ||
|---|---|---|---|---|---|---|---|---|---|---|
| Persons ('000) | ||||||||||
| Census count, place of usual residence | 8,072.2 | 6,503.5 | 5,156.1 | 1,781.5 | 2,660.0 | 557.6 | 232.6 | 454.5 | 25,422.8 | |
| Including people who have been missed | ||||||||||
| Plus – net undercount(b) | -2.6 | 20.9 | 50.2 | 17.2 | 83.3 | 8.9 | 15.0 | -2.8 | 190.1 | |
| Plus – demographic adjustment | 13.0 | 10.8 | 8.4 | 2.7 | 4.7 | 0.9 | 0.5 | 0.8 | 41.6 | |
| Plus – residents temporarily overseas | 13.2 | 10.8 | 5.9 | 1.4 | 3.0 | 0.5 | 0.9 | 0.9 | 36.6 | |
| Equals – ERP at Census night | 8,095.7 | 6,545.9 | 5,220.6 | 1,802.8 | 2,751.0 | 567.8 | 249.0 | 453.4 | 25,691.1 | |
| Backdating to 30 June 2021 | ||||||||||
| Minus – births | 11.5 | 8.7 | 7.3 | 2.1 | 3.7 | 0.7 | 0.4 | 0.6 | 35.0 | |
| Plus – deaths | 7.0 | 5.1 | 4.1 | 1.9 | 1.9 | 0.6 | 0.1 | 0.3 | 20.9 | |
| Minus – net interstate migration | -0.8 | -1.2 | 2.2 | -0.1 | 0.2 | -0.1 | -0.3 | 0.1 | .. | |
| Minus – net overseas migration | -1.8 | -4.6 | -2.5 | -0.6 | -0.9 | -0.1 | -0.1 | -0.5 | -11.1 | |
| Equals – ERP at 30 June 2021 | 8,093.8 | 6,548.0 | 5,217.7 | 1,803.2 | 2,749.9 | 567.9 | 249.2 | 453.6 | 25,688.1 | |
(a) Includes Other Territories comprising Jervis Bay Territory, Christmas Island, the Cocos (Keeling) Islands and Norfolk Island.
(b) Includes Census gross undercount and gross overcount from the PES, and minor adjustments to ensure coherence with ERP.
.. not applicable
Demographic adjustments are a range of adjustments made to the 2021 Census counts and PES estimates to ensure better internal consistency and coherence with:
The adjustments address issues such as:
| Original sex ratio | Adjusted sex ratio | Total population adjustment (‘000) | |
|---|---|---|---|
| 0 to 4 | 107.2 | 105.8 | 16.8 |
| 5 to 9 | 106.6 | 105.9 | 8.0 |
| 25 to 29 | 100.4 | 101.7 | 8.5 |
| 30 to 34 | 99.0 | 98.0 | 8.4 |
We adjusted the sex ratio for the Northern Territory from 101.5 to 103.3, with no change to the total population size.
We adjusted the population estimates of persons aged 95 and over using a survivor ratio method (see New estimates of Australia’s centenarian population). The survivor ratio method uses highly reliable deaths data from administrative sources, and combined with survivorship trends, produces more useful estimates of population at the highest ages. The adjustments were minor, also demonstrating the high quality of Census 2021 data at the highest ages.
Intercensal difference is the difference between the new Census-based estimate (known as rebased ERP) and the quarterly component-based estimate (known as unrebased ERP). We calculate it by state, sex, and cohort.
Intercensal difference may result from error in the start population estimate, the end population estimate and/or in the quarterly estimates of births, deaths, or migration. It is generally not possible to attribute intercensal difference to a particular source, or to quantify the error that each source contributes.
We revised quarterly population estimates between the current and previous Censuses by assuming intercensal difference accumulated evenly across the five-year period, except for certain cohorts in Tasmania and the Australian Capital Territory. For these cohorts, we allocated more intercensal difference to quarters before June 2020.
Our data confrontation showed that error in the state and territory allocation of migration estimates contributed to the relatively large intercensal difference in Tasmania and the ACT, for cohorts aged 20 to 35 at 30 June 2021. Given the international border closure in February 2020, and state border crossing restrictions, this error was more likely to have accumulated before then. The adjustment makes the growth rates over time more plausible.
For the 2016–2021 period, the preliminary intercensal difference was 52,211. We will produce final intercensal difference when we produce final rebased ERP, and revise all components of growth for the 2016-21 period.
| 1991 | 1996 | 2001 | 2006 | 2011 | 2016 | 2021(c) | |
|---|---|---|---|---|---|---|---|
| Persons ('000) | |||||||
| NSW | 2.4 | -15.8 | -27.0 | 10.5 | 45.4 | 3.6 | 94.3 |
| Vic. | 7.0 | -20.9 | 35.6 | -36.6 | 24.4 | -86.7 | 102.6 |
| Qld | 11.1 | 15.9 | 5.8 | -38.1 | 9.1 | 10.5 | 4.0 |
| SA | 10.4 | 4.5 | -9.6 | -13.5 | 3.4 | -3.7 | -29.5 |
| WA | 29.9 | -3.3 | 11.5 | -8.2 | -4.4 | 37.9 | -67.5 |
| Tas. | -6.3 | -1.1 | -1.8 | -1.0 | 0.3 | 7.2 | -26.5 |
| NT | -6.7 | -5.2 | -0.2 | -4.0 | -1.0 | 5.0 | -3.3 |
| ACT | 4.2 | -1.2 | -4.3 | -5.4 | 0.4 | 0.9 | -21.8 |
| Australia(b) | 51.9 | -27.5 | 10.6 | -96.0 | 77.7 | -24.9 | 52.2 |
| Percent of population | |||||||
| NSW | 0.0 | -0.3 | -0.4 | 0.2 | 0.6 | 0.1 | 1.2 |
| Vic. | 0.2 | -0.5 | 0.7 | -0.7 | 0.4 | -1.4 | 1.5 |
| Qld | 0.3 | 0.5 | 0.2 | -0.9 | 0.2 | 0.2 | 0.1 |
| SA | 0.7 | 0.3 | -0.6 | -0.9 | 0.2 | -0.2 | -1.7 |
| WA | 1.8 | -0.2 | 0.6 | -0.4 | -0.2 | 1.5 | -2.5 |
| Tas. | -1.4 | -0.2 | -0.4 | -0.2 | 0.1 | 1.4 | -4.9 |
| NT | -4.1 | -2.9 | -0.1 | -1.9 | -0.4 | 2.0 | -1.3 |
| ACT | 1.5 | -0.4 | -1.4 | -1.6 | 0.1 | 0.2 | -5.1 |
| Australia(b) | 0.3 | -0.2 | 0.1 | -0.5 | 0.4 | -0.1 | 0.2 |
(a) Negative numbers mean that the unrebased ERP underestimated the population. Positive numbers means that the unrebased ERP was an overestimate.
(b) Includes Other Territories. Excludes Other Territories prior to 2016.
(c) Preliminary intercensal difference.
| Persons ('000) | Percent of population | |
|---|---|---|
| 0-4 | 11.0 | 0.7 |
| 5-9 | 19.1 | 1.2 |
| 10-14 | -8.2 | -0.5 |
| 15-19 | 2.3 | 0.2 |
| 20-24 | 5.3 | 0.3 |
| 25-29 | 27.5 | 1.5 |
| 30-34 | 17.2 | 0.9 |
| 35-39 | -3.5 | -0.2 |
| 40-44 | -4.4 | -0.3 |
| 45-49 | -5.3 | -0.3 |
| 50-54 | -6.8 | -0.4 |
| 55-59 | -11.7 | -0.8 |
| 60-64 | -5.2 | -0.4 |
| 65-69 | -2.2 | -0.2 |
| 70-74 | -8.5 | -0.8 |
| 75-79 | 7.0 | 0.9 |
| 80-84 | 4.1 | 0.8 |
| 85 and over | 14.5 | 2.6 |
(a) Negative numbers mean that the unrebased ERP underestimated the population. Positive numbers means that the unrebased ERP was an overestimate.