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3228.0.55.001 - Population Estimates: Concepts, Sources and Methods, 2009  
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Contents >> Data sources >> Census undercount

CENSUS UNDERCOUNT

9.11 The level of coverage of the Australian Census is considered to be excellent, and compares favourably with censuses in other countries. Census data are used for a variety of purposes, without prior adjustment for undercounting. However, because population estimates are used in important ways such as government funding and electoral representation (paragraph 1.23), and given that the level of undercounting is related to important variables such as geographic area, age and sex which are used for population estimates, it was decided that as from 1971, for the purpose of estimating population, adjustments should be made to compensate for undercounting.

9.12 Census undercounting is measured primarily by the PES, a sample survey conducted immediately after the Census. While the first PES was run in 1966, the 1976 PES was the first to be used for population estimates. Although population estimates from 1971 onwards have taken account of the undercount, due to inadequacies in the 1971 PES the 1971 estimate was derived by working back from the 1976 estimate using the intercensal data on births, deaths and net overseas movement.

9.13 Note that in this publication, references to the undercount rate always refer to the net undercount rate and not the undercount adjustment factor. The undercount adjustment factor was developed in 2006 to be applied directly to Census counts to inflate them to a PES consistent number.

9.14 The undercount adjustment factor is defined in the Glossary of Census of Population and Housing - Details of Undercount, Aug 2006 (cat.no. 2940.0) as:

The undercount adjustment factor is the ratio of the PES population estimate to the Census count. This factor can be applied to the Census counts to indicate how many people should have been counted in the Census for that category.

9.15 Mathematically, this is expressed as:

      Equation: eq9_1

9.16 However, the net undercount rate as reported in previous PES summary results, expresses the net undercount as a percentage of the PES estimate of the number of people who should have been counted in the Census.

9.17 Mathematically, this has traditionally been defined as:
      Equation: eq9_2

9.18 In both cases, the PES estimate represents the weighted survey estimate of how many people
should have been counted in the Census whereas the Census count represents the number who were counted. The reason we have reported both is that the net undercount rate is referred to in the Commonwealth Electoral Act 1918.

9.19 Details of the 2006 survey are available in:
9.20 Interim population estimates as calculated using the PES are subject to adjustments based on demographic analysis ('demographic adjustments'). These demographic adjustments are explained in more detail in Appendix 1 - Demographic adjustment.


Census undercount in states/territories

9.21 As shown in Table 9.2, there is variation in the rate of undercount for the individual states and territories. New South Wales, Victoria, South Australia and the Australian Capital Territory have shown relatively small variations. In Queensland there has been a considerable and consistent increase - from 1.7% in 1996 up to 3.7% in 2006. In the Northern Territory the rate was relatively low in 1996 (2.9%) but has risen steadily to 7.6% in 2006. The Northern Territory has consistently shown the highest rate of undercount of all states and territories.

9.2 Net Undercount Rate, State/territory of usual residence - 1991 to 2006

1991 Census
1996 Census
2001 Census
2006 Census
%
SE
%
SE
%
SE
%
SE

New South Wales
1.9
0.1
1.5
0.2
2.0
0.2
2.4
0.4
Victoria
1.8
0.1
1.6
0.3
1.4
0.2
2.3
0.4
Queensland
1.8
0.1
1.7
0.3
1.9
0.2
3.7
0.4
South Australia
1.6
0.1
1.3
0.3
1.6
0.2
2.3
0.4
Western Australia
2.1
0.2
1.6
0.3
2.0
0.3
3.2
0.6
Tasmania
1.7
0.2
1.4
0.4
1.6
0.3
2.0
0.6
Northern Territory
2.9
0.7
3.1
1.6
4.0
0.6
7.6
1.5
Australian Capital Territory
1.4
0.2
1.1
0.3
1.0
0.4
1.2
1.0
Australia
1.8
0.1
1.6
0.1
1.8
0.1
2.7
0.2




Census undercount by age and sex

9.22 As has been observed in previous censuses in Australia, as well as censuses overseas, undercount is greatest in the young adult range, from approximately 15 to 35 years. Older adults are much more likely to be enumerated than younger adults.

9.23 At all ages undercounting is greater for males than females, averaging 3.3% and 2.1% respectively in 2006. The highest rate observed in 2006 was 8.1% for males aged 25-29. The highest rate for females was 6.0% for the 20-24 year age group.

9.3 Net undercount rate, Sex by age group - 2006

Males
Females
Persons
Age group (years)
%
SE
%
SE
%
SE

0-4
2.9
0.8
3.9
0.9
3.4
0.6
5-9
1.8
0.9
3.0
0.8
2.4
0.6
10-14
2.3
0.8
1.9
0.9
2.1
0.6
15-19
3.7
0.9
3.3
0.8
3.5
0.6
20-24
7.5
1.2
6.0
1.1
6.8
0.8
25-29
8.1
1.1
5.7
0.9
6.9
0.8
30-34
5.4
1.0
2.5
0.9
3.9
0.7
35-39
4.2
0.9
1.0
0.8
2.6
0.6
40-44
3.4
0.9
1.5
0.8
2.5
0.6
45-49
2.3
0.8
1.2
0.7
1.7
0.6
50-54
2.5
0.8
0.7
0.8
1.6
0.6
55+
0.6
0.3
0.3
0.3
0.5
0.2
Total all ages
3.3
0.2
2.1
0.2
2.7
0.2




Census undercount by marital status and country of birth

9.24 The 2006 undercount rates and their standard errors for the marital status categories and for selected countries of birth are shown in Table 9.4. To a certain extent the rates reflect the age structures of the various categories. For example, the never married category, which has the highest undercount rate in the marital status classification, includes a higher proportion of people in the 15-19, 20-24 and 25-29 year age groups, for whom the rate of undercounting is higher.

9.25 Similarly, in the country of birth classification, the population of the older source (of immigrants) countries, such as England and Scotland, have older age profiles and lower undercount rates. On the other hand, immigrants born in China and India include a higher concentration in the young adult age groups.

9.4 Net undercount rate, Marital status and country of birth - 2006

%
SE

Registered marital status
Never married(a)
4.6
0.3
Widowed, divorced or separated
1.2
0.5
Married
1.0
0.2
Country of birth
Australia
2.5
0.2
New Zealand
3.3
1.5
England
-1.7
0.8
Scotland
-0.3
2.2
Italy
1.7
1.5
Greece
3.0
2.7
Vietnam
4.4
2.8
Phillipines
5.5
2.9
China
11.5
2.7
India
11.7
3.5
Other overseas
3.6
0.6

(a) Includes those who are living with a de facto partner and have never been in a registered marriage.



Census undercount by Indigenous status

9.26 In 2006, the Indigenous undercount rate was significantly higher than the non-Indigenous undercount rate. For more information on Indigenous undercount see the Technical Note in Experimental Estimates of Aboriginal and Torres Strait Islander Australians, Jun 2006 (cat. no. 3238.0.55.001).

9.5 Net undercount rate, Indigenous origin - 2006

Rate %
Standard Error

Indigenous
6.5
2.4
Non-Indigenous
2.6
0.2







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