3.4 Population counts and age - sex distributions
An independent view of the quality of statistical outputs from the 2021 Census of Population and Housing
Overview
This section provides comparisons of the age distributions of the population recorded in the 2021 Census for Australia and for states and territories with the corresponding age distributions estimated in the unrebased Estimated Resident Population at 30 June 2021.[1] It should be noted that the reference periods for the Census and the Estimated Resident Population differ, with the Census reference period some six weeks after that of the population estimates. Census counts are not backdated in this analysis.
Comparing the age distributions from the two sources provides information about the quality of both sources. The more alignment there is between the two and an understanding of the differences between them provide an indication of coherence and quality.
Both the Census and the Estimated Resident Population are potential sources of any difference observed between the two measures. The ABS investigates these differences and using data from the Census and from the Post Enumeration Survey, adds in numbers of Australian usual residents who were overseas on Census night, then re-estimates the Estimated Resident Population. This is referred to as rebased Estimated Resident Population. This rebased 2021 Estimated Resident Population will be released on 28 June 2022 and will form the basis of all population estimates from 2021 until the next Census in 2026.
Error in the Estimated Resident Population can arise from quality and coherence issues associated with the administrative data sources used for quarterly estimates. These can include delays in the reporting and registering of births and deaths, the use of preliminary estimates of net overseas migration and the quality of information collected on Incoming Passenger cards, and lags, quality and undercoverage of people reporting changes to their usual residence within Australia.
While some delay in registrations is observed each year across all jurisdictions, the pandemic has had a noticeable effect in increasing the delay of birth registrations, most particularly in Victoria where, for the June 2021 quarter, more than half of the records were received after the reference quarter. These delayed registrations are not included in the published Estimated Resident Population for 30 June 2021.
The Census is particularly useful in recalibrating or rebasing the Estimated Resident Population for geographic areas within Australia every five years, as the Estimated Resident Population is affected by the accuracy of measurement of people changing their place of usual residence in Australia which is based largely on Medicare data. Evidence shows that the pandemic and subsequent lockdowns have significantly affected intra and interstate movements over the last two years. The success of the national COVID-19 vaccination program has had a positive impact on the quality of administrative data used to estimate Australia’s state and territory population as the rollout of the vaccination program has seen large numbers of Australians updating their address information. However, this happened towards the end of the inter-censal period so would have been of limited benefit to the Estimated Resident Population estimates for 30 June 2021.
The Census counts used in this section eliminate overseas visitors before the comparison with Estimated Resident Population is made. A challenge in this regard is that the Census definition of a usual resident is not precisely the same as the definition of a usual resident in the Estimated Resident Population calculation. 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. A usual resident for the Estimated Resident Population is a person who is in Australia for 12 months over a 16-month period around the reference date. However, significantly reduced levels of overseas migration in the 18-month lead-up to the Census will have reduced the impact of this difference between the two datasets.
The comparisons shown below are based on the 2021 Census before any adjustment is made for the 2021 Post Enumeration Survey. This means, for example, that the people that the Post Enumeration Survey showed to have been incorrectly added (imputed) to non-responding dwellings (see Section 3.2, Table 3.2.2) are not removed from the Census counts before the comparisons with Estimated Resident Population are made. This approach is taken because the published Census data also include these incorrectly attributed people, although in 2021 it is known that there were fewer imputed people, and occupancy determination was more accurate.
Taking account of the difference outlined above, there is generally a good match between the 2021 Census and the 2016-based Estimated Resident Population for 30 June 2021 but the Census results for some of the smaller states and territories indicate there is room to improve the way administrative data is used between censuses to estimate the state or territory of residence and interstate movement for some recently arrived migrants.
[1] The unrebased Estimated Resident Population for 2021 is obtained by updating the 2016 rebased Estimated Resident Population using births, deaths, internal migration and international migration between 2016 and 2021. Unless otherwise specified, all references to Estimated Resident Population in this section refer to first released unrebased Estimated Resident Population.
3.4.1 Comparisons of the Census and the unrebased Estimated Resident Population by age, 2021 and 2016
Australia
For Australia (Figure 3.4.1), the peaks and troughs of the unrebased Estimated Resident Population age distribution are mirrored by the 2021 Census data. Note that the comparison of the two sources for Australia is up to 100 years of age, while for the states and territories it is truncated at 85 years because the numbers get much smaller after this age. ABS will make corrections at older ages as part of the recalculation of the Estimated Resident Population.
Numerically, the 2021 Census and the Estimated Resident Population are very close from age 40 onwards. Furthermore, for the under 40 years population, the 2021 Census counts are a closer fit to the Estimated Resident Population than was the case in 2016.
The largest differences between the two measures are in the childhood years (0-13 years of age) and the young adult years (20-35 years). At these ages, the Census results are lower than the Estimated Resident Population. This means that fewer people were counted in the 2021 Census in these age groups than would have been expected using the Estimated Resident Population. In the first age group, the largest differences were observed for children aged five years (4.7% lower in the Census) followed by babies (0 year olds), at 4.5% lower.
Notes: Unrebased Estimated Resident Population based on the 2016 Census as published on 16 December 2021.
Census count excludes overseas visitors. Includes Other Territories.
Notes: Unrebased Estimated Resident Population based on the 2011 Census.
Census count excludes overseas visitors. Includes Other Territories.
The Estimated Resident Population in the early childhood years is likely to be quite accurate because it is based on the number of births that occurred in the few years prior to the Census although, as noted earlier, an increased delay in birth registrations due to the pandemic is likely to have had an effect in Victoria. The omission of children at very young ages is a common feature of censuses not only in Australia, but also in other countries. However, in Australia there appears to be an undercount of children up to the age of 13 and this is more difficult to explain. Part of the explanation may lie in the different reference periods of the Census compared to the Estimated Resident Population, when some children would have had a birthday between the end of June and 10 August, causing an ‘age mismatch’ between the two data sources. An additional reason might be the movement of children between households as part of custodial arrangements where the children may not have been reported in the Census by either parent. The undercount of young adults (especially males) is also commonly observed in censuses because people of these ages are generally harder for censuses and surveys to contact. The Post Enumeration Survey confirms that the net undercount rate is relatively high for children aged 10-14 years, with the highest net undercount observed particularly for young men in their early 20s and early 30s.
States and territories
The pattern of differences observed for Australia between the Census and the unrebased Estimated Resident Population is similar across the states and territories, as is the closer alignment of the 2021 Census counts to the Estimated Resident Population compared with 2016. There are generally lower counts of babies, young children, and young adults in the Census and a much closer fit for ages 40 and over between the Census and the Estimated Resident Population. However, there are noticeable differences across the states and territories in proportional terms (see Figures 3.4.3 to 3.4.18).
In New South Wales, there were 6.1% fewer babies in the Census count than in the Estimated Resident Population, while for 26-29 year olds the Census counts were between 5% and 6% lower. Conversely, in Victoria, there were 4.2% more babies in the Census count than in the Estimated Resident Population, and this reflects the delayed registration of births in Victoria due to COVID-19, which in turn has had an impact on the Estimated Resident Population for that age. The largest difference for Victoria was for young adults, with the Census counts for 25-27 year olds more than 8% lower.
South Australia’s Census counts were closely aligned with the population estimates, with 4.3% fewer babies. In South Australia, the count of people aged 19 and 20 years also showed differences with the estimates (3% lower than the population estimates). Census counts in the older ages (65-77 years) were somewhat higher than for the Estimated Resident Population.
In Queensland and Western Australia, there were larger difference in the count of babies, which were 8.9% and 8.7% lower than the Estimated Resident Population respectively, but there were relatively small differences in the counts of young adults compared with the estimates. For young adults in Queensland, the largest proportional difference between Census counts and the population estimates were for those aged 26-28 years where the counts were between 4% and 4.5% lower. In Western Australia, as in South Australia, the Census counts of people aged 19 and 20 years showed the largest difference in the young adult cohort with the counts about 3.5% lower.
In Tasmania and the Australian Capital Territory, there was an unusual pattern in that the Census counts for young adults were much higher than that of the Estimated Resident Population. In Tasmania, the Census counts for the 27 to 32 age group were at least 15% higher than that of the Estimated Resident Population, while in the Australian Capital Territory, the difference was even more pronounced with the Census counts for those aged 18, 19, 26, 27 and 28 years, in particular, more than 20% higher than the population estimates.
This unusual pattern could be due to a number of factors such as the potential movement of students back to their parent’s homes during COVID-19 lockdowns, but early analysis suggests it also reflects inaccuracies in capturing state or territory of residence and/or interstate migration of new migrants to these two jurisdictions. This unusual pattern will be investigated during the construction of the rebased Estimated Resident Population to be published on 28 June 2022. At this stage, it seems highly likely that the unrebased Estimated Resident Population has heavily underestimated the young adult population in Tasmania and the Australian Capital Territory. This provides an example of the value of a five-yearly Census in improving population estimates, especially at the sub-national level.
For the Northern Territory, the most striking variations are the relatively large differences for babies (where the Census count was 18% lower than the population estimate), and for children and young people up to the age of 21 years. As indicated by the Post Enumeration Survey, this is likely to be due to Census undercount, especially of Aboriginal and Torres Strait Islander peoples. The Estimated Resident Population for the Northern Territory is also noticeably higher than the Census count for those in their thirties.
Notes: Unrebased Estimated Resident Population based on the 2016 Census as published on 16 December 2021.
Census count of usual residents of New South Wales, which excludes overseas visitors.
Notes: Unrebased Estimated Resident Population based on the 2011 Census.
Census count of usual residents of New South Wales, which excludes overseas visitors.
Notes: Unrebased Estimated Resident Population based on the 2016 Census as published on 16 December 2021.
Census count of usual residents of Victoria, which excludes overseas visitors.
Notes: Unrebased Estimated Resident Population based on the 2011 Census.
Census count of usual residents of Victoria, which excludes overseas visitors.
Notes: Unrebased Estimated Resident Population based on the 2016 Census as published on 16 December 2021.
Census count of usual residents of Queensland, which excludes overseas visitors.
Notes: Unrebased Estimated Resident Population based on the 2011 Census.
Census count of usual residents of Queensland, which excludes overseas visitors.
Notes: Unrebased Estimated Resident Population based on the 2016 Census as published on 16 December 2021.
Census count of usual residents of South Australia, which excludes overseas visitors.
Notes: Unrebased Estimated Resident Population based on the 2011 Census.
Census count of usual residents of South Australia, which excludes overseas visitors.
Notes: Unrebased Estimated Resident Population based on the 2016 Census as published on 16 December 2021.
Census count of usual residents of Western Australia, which excludes overseas visitors.
Notes: Unrebased Estimated Resident Population based on the 2011 Census.
Census count of usual residents of Western Australia, which excludes overseas visitors.
Notes: Unrebased Estimated Resident Population based on the 2016 Census as published on 16 December 2021.
Census count of usual residents of Tasmania, which excludes overseas visitors.
Notes: Unrebased Estimated Resident Population based on the 2011 Census.
Census count of usual residents of Tasmania, which excludes overseas visitors.
Notes: Unrebased Estimated Resident Population based on the 2016 Census as published on 16 December 2021.
Census count of usual residents of the Northern Territory, which excludes overseas visitors.
Notes: Unrebased Estimated Resident Population based on the 2011 Census.
Census count of usual residents of Northern Territory, which excludes overseas visitors.
Notes: Unrebased Estimated Resident Population based on the 2016 Census as published on 16 December 2021.
Census count of usual residents of the Australian Capital Territory, which excludes overseas visitors.
Notes: Unrebased Estimated Resident Population based on the 2011 Census.
Census count of usual residents of Australian Capital Territory, which excludes overseas visitors.
People aged 85 years and over
Table 3.4.1 indicates the difference in the 2021 Census counts of people aged 85 years and over when compared with the unrebased Estimated Resident Population for Australia and the states and territories. In 2016, there were 2,251 more people aged 85 and over in the Census counts than in the Estimated Resident Population, while in 2021 there were 6,375 fewer in the Census. This may reflect the difficulties encountered in 2021 in enumerating people living in aged care and in nursing homes in those areas experiencing lockdown. Nevertheless, the numbers in the Census and the Estimated Resident Population are close.
| Unrebased ERP | Census | Difference | % difference |
|---|---|---|---|---|
| 2021 | ||||
New South Wales | 185,954 | 183,895 | -2,059 | -1.1 |
Victoria | 144,764 | 142,475 | -2,289 | -1.6 |
Queensland | 98,976 | 97,140 | -1,836 | -1.9 |
South Australia | 47,085 | 47,325 | 240 | 0.5 |
Western Australia | 50,563 | 50,106 | -457 | -0.9 |
Tasmania | 13,053 | 12,995 | -58 | -0.4 |
Northern Territory | 1,268 | 1,157 | -111 | -8.8 |
Australian Capital Territory | 6,928 | 7,181 | 253 | 3.7 |
Australia (a) | 548,717 | 542,342 | -6,375 | -1.2 |
2016 | ||||
New South Wales | 168,420 | 167,506 | -914 | -0.5 |
Victoria | 124,825 | 127,993 | 3,168 | 2.5 |
Queensland | 84,923 | 85,528 | 605 | 0.7 |
South Australia | 44,122 | 44,479 | 357 | 0.8 |
Western Australia | 43,608 | 42,420 | -1,188 | -2.7 |
Tasmania | 11,595 | 11,767 | 172 | 1.5 |
Northern Territory | 1,029 | 938 | -91 | -8.8 |
Australian Capital Territory | 6,057 | 6,158 | 101 | 1.7 |
Australia (a) | 484,591 | 486,842 | 2,251 | 0.5 |
(a) Australia includes Other Territories.
Small area geographies
One of the most important reasons for the Census is to obtain population estimates by age in small geographic areas. Between censuses, the Estimated Resident Population is prepared for sub-state geographies but these estimates are subject to more error than the state and/or territory estimates because data on where international migrants settle is weak and local area populations can be substantially affected by internal migration, which is also difficult to measure precisely. A vital contribution of the Census is to rebase the Estimated Resident Population every five years for small area geographies. Therefore, while gross comparisons can be made between the Census counts and the Estimated Resident Population for small areas, assessing the meaning of the differences requires considerably more analysis than was possible in the time available to the Panel. This work will be carried out by the ABS. In addition, the Post Enumeration Survey does not provide reliable estimates of the undercount or overcount below the level of capital city and rest of state.
3.4.2 Sex ratio
The ratio of males to females in each age is a conventional way of measuring the accuracy of census data. Figure 3.4.19 compares the age-specific sex ratios (males per 100 females) of the 2021 Census and the unrebased Estimated Resident Population at 30 June 2021. The differences between the two sources are very small with the Estimated Resident Population sex ratios being slightly higher over a wide range of adult ages (see Figure 3.4.19), reflecting a slightly higher undercount of males in the Census compared with females.
Notes: Unrebased Estimated Resident Population based on the 2016 Census as published on 16 December 2021.
Census count excludes overseas visitors and includes Other Territories.
3.4.3 Implications for Census quality
Including the imputed records, the Census and the Estimated Resident Population comparisons are reasonably close from age 40 upwards. Overall, there is a much closer fit between the Census counts and the Estimated Resident Population in 2021 compared to 2016. However, for babies, the childhood years and young adults, the Census estimate is lower. This pattern is true for most states and territories except for Tasmania and the Australian Capital Territory, which saw higher counts of young adults in the Census than in the population estimates; and in Victoria, which saw more babies (0 years of age) counted in the Census because of delays in birth registrations. Despite these differences, and in light of the close alignment overall, we conclude that the Census is fit-for-purpose for its very important use in updating the Estimated Resident Population by age and sex at the national and state/territory levels.