2940.0 - Census of Population and Housing - Details of Undercount, 2011  
ARCHIVED ISSUE Released at 11:30 AM (CANBERRA TIME) 21/06/2012   
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1 The aim of the Census is to count every person who spent Census night in Australia, with the exception of foreign diplomats and their families. Visitors to Australia are in scope regardless of how long they have been in the country or how long they plan to stay. Australian residents out of the country on Census Night are out of scope in the Census.

2 The scope of the Census includes Australian residents in Antarctica and people in the territories of Jervis Bay, Cocos (Keeling) Islands and Christmas Island. The other Australian External Territories, Norfolk Island and minor islands such as Heard and McDonald Islands, are outside the scope of the Australian Census. People outside Australia who are not required to undertake migration formalities, such as those on oil and gas rigs off the Australian coast, are in scope.

3 All private dwellings, except diplomatic dwellings, are included in the Census, whether occupied or unoccupied. Caravans in caravan parks, manufactured homes in manufactured home estates and self-care units in accommodation, for the retired or the aged, are counted only if occupied. Occupied non-private dwellings, such as hospitals, prisons, hotels, etc., are also included.

4 Details about the 2011 Census content, collection operations, confidentiality and privacy protection, processing and evaluation activities are contained in 2011 Census Nature and Content (cat. no. 2008.0).


Usual residence

5 The Australian Census counts people where they stayed on Census night. This means that the Census was conducted on an 'actual location' or 'place of enumeration' basis. There is, however, a need for data based on place of usual residence, and Census counts are available on this basis.

6 For usual residents of Australia, 'place of usual residence' for the 2011 Census is defined as the address at which a person has lived or intends to live for six months or more in 2011. While for most people their usual residence was the same as their actual location on Census night, some people spent Census night at a place other than where they usually lived. Thus, their 'place of enumeration' and their 'place of usual residence' were different.

7 People visiting Australia on Census night are included in the Census counts on a place of enumeration basis but not those on a place of usual residence basis.

8 Usual residents of Australia who are temporarily overseas on Census night are not included in Census counts on either a place of usual residence or place of enumeration basis. However, counts of these people are accounted for in the estimated resident population of Australia (ERP). For information on the calculation of ERP, see the ABS publication Australian Demographic Statistics, December quarter 2011 (cat. no. 3101.0), released on 20 June 2012.

9 Estimates presented in this publication are on a place of usual residence basis (unless otherwise noted).


10 Census counts include persons whose Census form was partially completed. During Census processing, values for age, sex, marital status, and state/territory of usual residence are imputed if they were left blank on the Census form. Values for all other variables left blank (e.g. Indigenous status, country of birth) are set to not-stated or not applicable, depending on the (possibly imputed) age of the person.

11 Census counts also include imputed person records for non-responding dwellings (for further information around imputed persons see Components of net undercount and the Identifying Census late returns Technical Note). These records have values imputed for age, sex, marital status, and place (CLW, SA1 and State/territory) of usual residence. Values for all other variables (e.g. Indigenous status, Country of birth) are set to not-stated or not applicable, depending on the imputed value for age.

12 Therefore, care should be taken when interpreting undercount estimates for Indigenous status and country of birth. For example, there were 1,058,447 persons (4.9% of the Census count) whose Indigenous status was not stated in the 2011 Census. These persons do not contribute to either the Indigenous or non-Indigenous Census count, although they do contribute to total Census counts by age, sex, marital status, and state/territory of usual residence.

Resolution of Census not-stated value

13 For Census purposes, age, sex, marital status and state of usual residence are imputed (statistical process for predicting values where no response was provided) during Census processing where these items have been left blank, including where a whole person record has been imputed. Missing values for any other items remain 'not-stated' in the final version of Census counts.

14 The PES uses Census data items to form benchmark categories for weighting and estimation purposes. Two such data items are Indigenous status and country of birth. In cases where these items have been left blank in the Census, a value was imputed during PES processing so that these items could be used for 2011 PES benchmarks.

15 The imputation method involved imputing both variables together. This required the assumption that anyone who is imputed as Indigenous is born in Australia.

16 For benchmarking purposes, 12 categories of persons were defined:

  • Indigenous, Australian-born
  • non-Indigenous, Australian-born
  • non-Indigenous, born in other country - 1 to 9 ranked (in terms of population) countries of birth from the 2011 Census
  • non-Indigenous, born elsewhere.

17 The full range of 10 non-Australian country of birth classes were used in benchmarking at the Australia level; for most regions in Australia they were collapsed to 2 classes.

18 These are non-overlapping categories, but persons with not-stated values in the Census may be imputed as partially in two or more of the categories (with the sum of all categories adding to 1 for each person).

19 Imputation was performed separately for each person within non-overlapping imputation classes, whereby data are imputed by the proportion of stated values of respondents in the same imputation class. In order of increasing importance, these classes were:
  • SA2 (Geographic unit averaging 10,000 persons; see Glossary);
  • Census form type;
  • Age Group - 5 year age groups to 75+; and
  • Sex.

Census response type

20 Although all persons resident in Australia should have been counted in the Census, not all dwellings would have received a Census form. This is because not all dwellings were habitable or, in the case of a diplomatic dwelling, did not contain people within scope of the Census.

21 Census defines ‘private dwellings’ as structures established for self-contained accommodation, which may be ‘occupied’ or ‘unoccupied’, where:
  • ‘Occupied’ refers to private dwellings that were occupied by one or more persons on Census night. Non-permanent, removable structures such as caravans, tents, manufactured homes or houseboats were treated as private dwellings and included in the Census only if they were occupied on Census night. Occupied improvised dwellings, such as sheds and garages, were also included in the Census.
  • 'Unoccupied' refers to private dwellings capable of being lived in but which were not occupied on Census night. This includes unoccupied holiday houses, vacant houses to let, and unoccupied apartments. Unoccupied non-permanent or improvised structures, such as caravans, converted garages, tents, manufactured homes and houseboats, were not counted in the Census. Houses under construction and derelict houses were also not included.

22 Private dwellings may also be classified as a ‘non-contact’ or ‘refusal’ in the Census, where:
  • 'Non-contact' refers to private dwellings where the Census collector was unable to make contact with a householder within five visits, and was unable to verify that the dwelling was unoccupied on Census night.
  • 'Refusal' refers to private dwellings where the householder refused to participate in the Census.

23 For PES purposes, dwellings that provided a Census return after the commencement of PES enumeration were flagged as 'late returns' and treated accordingly during PES processing. For information around the treatment of late returns, see the Identifying Census late returns Technical Note.

24 During Census processing, the number of usual residents for non-contact and refusal dwellings was imputed, as well as some personal characteristics.

Census Data Quality

25 Census data are subject to a number of inaccuracies resulting from errors by respondents or mistakes in collection or processing. Whilst many of these are corrected by careful processing procedures, some still remain. The effect of the remaining errors is generally slight, although it may be more important for smaller groups in the population.

26 The main kinds of errors occurring in the Census are:
  • Partial non-response: In some cases where an answer is not provided to a question, an answer is imputed (often from other information on the form). In other cases, a 'not stated' code is allocated.
  • Processing error: While such errors can occur in any processing system, quality management is used continuously to improve the quality of processed data, and to identify and correct data of unacceptable quality.
  • Respondent error: Because processing procedures cannot detect or repair all errors made by people in completing their forms, some remain in final data; and
  • Undercount.

27 For further information on sources of error in the Census, refer to the appropriate entries in the 2011 Census Dictionary (cat. no. 2901.0), released on 23 May 2011.


28 The three major changes to the 2011 PES were the following:
  • Introduction of Automated Data Linking (ADL) to complement the existing clerical Match and Search System. For more information about ADL see Linking and Matching.
  • Expansion of the PES sample to incorporate an increased focus on enumeration of discrete Indigenous communities and focused sampling in parts of the mainstream sample for which there was a relatively high proportion of Indigenous persons. Enhancements to questionnaires (in particular the removal of a screening question, to improve Indigenous identification), field materials and training were also made in support of the sample design. For more information about the 2011 PES sample and sampling procedures see Survey Enumeration.
  • Transition to the Australian Statistical Geography Standard (ASGS), which replaced the Australian Standard Geographical Classification (ASGC) as the framework for PES and Census geography.

Impact of the transition to the ASGS

29 With the geographic classification used in the Census changing from the Australian Standard Geographical Classification (ASGC) to the Australian Statistical Geography Standard (ASGS), a mixture of geographies was used in the 2011 PES, with different geographic units used at different stages of the survey cycle. This development presented challenges for the ABS in relation to the sample design, however these challenges were well managed throughout the complete PES cycle.

30 As in 2006, the 2011 PES sample design was structured by CDs. This is in contrast to the 2011 PES field operations, processing and output, which all used geographical units from the ASGS, primarily Statistical Area Level 1 (SA1). To overcome this challenge, the PES sample was recoded to the ASGS geography prior to the start of enumeration.

31 The second challenge posed was the spatial definition of discrete Indigenous communities, some of which differed between the two geography frameworks. This was managed by a mapping exercise which compared the two geographies for the 28 communities selected in the PES sample.


Mainstream private dwelling sample

32 The mainstream sample of private dwellings was derived from the 2006 Monthly Population Survey Parallel Sample frame, which is based on the Statistical Division and Subdivision structure of the Australian Standard Geographical Classification (ASGC). This sampling framework first divides Australia into 100 geographical areas. These areas are then divided into strata according to population density, remoteness and growth, then:
  • in the first stage of selection, a sample of Census collection districts (CDs) is selected (systematically, with probability proportional to size) to represent each stratum;
  • in the second stage of selection, each selected CD is divided into smaller areas called blocks, one of which is selected (again systematically, with probability proportional to size); and
  • in the third stage, a sample of dwellings in the selected block is taken using systematic equal probability sampling.

33 In less populated areas, an additional stage precedes the selection of CDs to ensure the sample is not too geographically spread (as that would lead to unacceptable enumeration costs).

34 The 2011 PES sample departed from the Parallel Sample by selecting extra dwellings, and in some cases extra CDs, in areas with higher proportions of Indigenous households. This measure was introduced in the 2011 PES to improve estimates of Indigenous undercount.

Discrete Indigenous communities sample

35 CDs with an identified Indigenous population of greater than 75% are classified differently from the remaining population for both operational and sampling reasons. These CDs and the Indigenous communities within them form the Indigenous Communities frame (ICF).

36 For the purposes of PES sampling, discrete Indigenous communities were grouped into 'sets' comprising main communities and their associated outstations. The selection of main communities for the 2011 PES was undertaken with probability proportional to the size of the set. The aim was to select as representative a sample as possible while also considering cost constraints, reasonable interviewer workloads and expected sample size.

37 Where a community was selected in the PES sample, selection of dwellings within that community followed the same procedure as for the selection of private dwellings within selected blocks in the non-ICF sample component. That is, the interviewer compiled a list of all the dwellings within the community and a sample was taken using systematic equal probability sampling.

38 A selection of outstations associated with each selected main community was also included in the sample. Each outstation had an equal chance of selection and, once selected, all dwellings within the outstation were enumerated.

Sampling strategies to improve Indigenous population estimates

39 Initial attempts to improve the quality of Indigenous undercount information occurred in the 2006 PES when remote areas and discrete Indigenous communities were enumerated for the first time. The 2011 PES strived to further improve the estimates of Indigenous undercount by increasing the number of discrete Indigenous communities selected (28 in 2011 compared with 20 in 2006), as well as the total number of dwellings selected within the community sample (around 700 dwellings in 2011, compared with 350 in 2006). As predicted, this increase in sample resulted in a decrease in the sampling error of estimates of Indigenous undercount.

40 In addition to the increase in the discrete Indigenous communities sample, the 2011 PES increased the number of selected dwellings from areas outside the discrete Indigenous communities that were identified as having a relatively high proportion of Indigenous persons. This focused sampling of additional areas within the mainstream sample was a key sample design change from 2006.

41 The focused sampling approach involved a combination of extra dwellings selected from within the parallel sample, as well as some additional selections from CDs outside the parallel sample. This focus on obtaining an increase in Indigenous respondents within the mainstream sample was of particular importance, given approximately three-quarters of the Aboriginal and Torres Strait Islander population usually reside outside of discrete communities. This strategy was successful in increasing the number of Indigenous respondents in the PES in 2011 (6,401 persons, compared with 3,354 persons in 2006).

Reliability of undercount estimates

42 As the estimates of undercount are based on data from a sample survey, they are subject to sampling error. Some of the estimates presented in this publication have high SEs, and these estimates should be used with caution. The estimates are also subject to non-sampling error. For more information about sampling and non-sampling error see the Sampling Error Technical Note.