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Understanding Census data quality

There are a number of ways in which the quality of Census data can be assessed. Response rates (for both dwellings and people) are measures that are used internationally and are an important part of understanding quality, as is item non-response. In addition, the Post Enumeration Survey provides an independent measure of Census coverage through its estimate of undercount and overcount.

The key indicators for the 2016 Census of Population and Housing support that the Census data is of high quality, with high response rates (95.1%) and low levels of net undercoverage (1.0%).

Dwelling response rate

The dwelling response rate measures the number of private dwellings that returned a completed Census form as a proportion of all private dwellings believed to be occupied on Census night. The dwelling response rate for the 2016 Census was 95.1%. In the 2011 Census this response rate was 96.5%, and in 2006 it was 95.8%. The table below provides a breakdown of dwelling response by state and territory across the last three Censuses.

The decline in the reported dwelling response between the 2011 and 2016 Censuses is partly due to changes in the collection method (see How we collected your information for more details) that resulted in an overestimate of the number of occupied private dwellings. As a result, this has led to a lowering of the overall dwelling response rate. Despite the slight decline, the dwelling response rate is very high, and will produce high quality data.

Dwelling response rate by State and Territory, 2006 - 2016

	2006	2011	2016
New South Wales	95.5%	96.4%	95.1%
Victoria	96.3%	96.8%	95.3%
Queensland	95.7%	96.4%	94.7%
South Australia	97.0%	97.5%	96.3%
Western Australia	95.1%	95.9%	94.6%
Tasmania	97.4%	97.8%	95.6%
Northern Territory	91.6%	92.4%	90.7%
Australian Capital Territory	96.3%	97.1%	95.8%
Australia*	95.8%	96.5%	95.1%

Person response rate

The person response rate measures how many people are included on a returned Census form as a proportion of all people (responding and non-responding) believed to be in Australia on Census night. Private dwellings believed to be occupied but did not return a Census form, contribute to the numbers of people who are considered non-responding. Similarly, people believed to be present in non-private dwellings (hotels, hospitals, boarding houses, etc.) but who did not complete a Census form contribute to the numbers of non-responding people.

There are a number of reasons why person non-response occurs in the Census. People may indicate a desire to mail back a Census form or to complete the form online but may forget to do so, some people may refuse to complete a Census form, and some may have been left off a form.

The dwelling response rate (outlined in the previous section) is only calculated for private dwellings, while the person response rate includes all people regardless of whether they are in a private or non-private dwelling,

The person response rate was 94.8% in the 2016 Census. In the 2011 Census it was 96.3% and in the 2006 Census is was 95.8%. Like dwelling responses rates, the over identification of occupied homes have led to a lowering of the reported person response rate in 2016 compared to 2011 and 2006.

Where people are believed to have not responded to the Census, data are imputed for them (see Imputation section in Turning your information into Census data) including selected demographic characteristics such as age, sex, marital status and usual address. This ensures a full and accurate dataset is produced.

Person response rate by State and Territory, 2006 - 2016

	2006	2011	2016
New South Wales	95.7%	96.4%	95.0%
Victoria	96.4%	96.8%	95.2%
Queensland	95.5%	95.9%	94.2%
South Australia	97.1%	97.4%	96.1%
Western Australia	94.9%	95.2%	94.2%
Tasmania	97.3%	97.7%	95.5%
Northern Territory	91.9%	92.3%	89.4%
Australian Capital Territory	96.3%	96.7%	95.4%
Australia*	95.8%	96.3%	94.8%

Item non-response rates

The majority of item non-response is attributable to the people who did not respond to the Census at all. These people have records created for them (see Imputation section in the Turning your information into data chapter) and whilst a small number of data items are imputed for them (age, sex, marital status and usual address), the remaining data items are left as not stated.

The second and smaller contributor to item non-response is when people return a Census form but may not answer a particular question(s). This is referred to as item non-response (excl. imputed persons).

Item non-response rates are a measure of how many people did not respond to a particular question as a proportion of the total number of people the question was applicable to. In this instance the response is left as not stated.

This graph shows the contribution of imputed records to item non-response, using Country of Birth (BPLP) as an example.

There is a marked difference between item non-response rates for people who completed their form online versus those who completed the paper form. The Census online form makes a subset of questions mandatory which means respondents cannot submit a blank answer to specific questions. The online form is also quicker to complete, and smart features such as using names to assist with subsequent questions and sequencing the respondent only to necessary questions makes the online form easier to complete. Through these factors, people responding online tend to answer more questions.

The increase in online participation in the 2016 Census has pleasingly reduced the number of questions left blank in completed Census forms.

This table shows item non-response (excluding imputed records) for Country of Birth and illustrates the majority contribution to non-response from paper form submissions.

	Paper form	Online form
New South Wales	91.9%	8.1%
Victoria	93.0%	7.0%
Queensland	93.3%	6.7%
South Australia	95.1%	4.9%
Western Australia	92.1%	7.9%
Tasmania	97.2%	2.7%
Northern Territory	95.8%	4.1%
Australian Capital Territory	86.2%	13.8%
Australia*	92.9%	7.1%

• Estimates of undercount and overcount provide users with an assessment of the completeness of Census counts and a measure of Census imputation performance. As such, undercount and overcount can be used to evaluate the effectiveness of Census collection procedures and data processing, so improvements can be made for future Censuses.
  
• Net undercount is a key component in rebasing the Estimated Resident Population (ERP) for 30 June of the Census year. Rebasing is done by adding the net undercount from PES to the new Census population counts, before applying further demographic adjustments in order to recalibrate ERP to the latest Census.

Please refer to Census of Population and Housing - Details of Undercount, 2016 for details.

The PES helps to measure the quality of the Census and ensure the quality of Australia’s population estimates.

The net undercount rate in 2016 was 1.0% (equivalent to 226,407 persons; i.e. the PES population estimate was 226,407 more persons than was counted in the 2016 Census, compared with 1.7% in 2011. The lower net undercount rate in 2016 was driven by a large positive difference in the number of people who were counted on multiple Census forms or counted in error and in the number of persons imputed into non-responding Census dwellings in 2016, compared with 2011. These larger changes in overcount (including over-imputation in the Census) have offset the smaller positive difference in the number of people who were missed from responding Census dwellings.

The 2016 PES estimated a net undercount rate of 17.5% for Aboriginal and Torres Strait Islander peoples (equivalent to 137,750 persons who were missed in the Census). This is only slightly higher than 2011, which estimated a net undercount of 114,188 persons, or a rate of 17.2%.

For Country of birth, the net undercount rate for persons born overseas was lower in 2016 (7.0%) compared with 2011 (8.8%), while persons born in Australia had a higher net undercount rate (8.1% in 2016 compared with 6.6% in 2011). The largest changes in net undercount for the ten highest ranked countries (in terms of population residing in Australia) were for China (from 14.9% in 2011 to 6.2% in 2016), India (from 9.7% in 2011 to 5.0% in 2016) and the Philippines (from 9.1% in 2011 to 5.5% in 2016).

Census Independent Assurance Panel

An independent panel of eminent Australian and international statisticians, academics, and state government representatives was established to independently review and assure the quality of statistical outputs from the 2016 Census.

Overall, the panel found 2016 Census data to be fit for rebasing the Estimated Resident Population and having comparable quality to previous Australian Censuses and International Censuses. The panel concluded that 2016 Census data can be used with confidence.

Their report can be found here.

Quality Declaration

This quality declaration details how the Census addresses each of the elements of the ABS’ quality framework, covering relevance, timeliness, accuracy, coherence, interpretability and accessibility.

Institutional environment

For information on the institutional environment of the ABS, including the legislative obligations of the ABS, financing and governance arrangements, and mechanisms for scrutiny of ABS operations, please see ABS Institutional Environment.

Relevance

The Australian Census of Population and Housing is the official count of population and dwellings and collects details of age, sex and other characteristics of the population.

The Census aims to measure the number and key characteristics of dwellings and people in Australia on Census Night. All people in Australia on Census Night are in scope, except foreign diplomats and their families. Visitors to Australia are counted regardless of how long they have been in the country or how long they plan to stay. Australian residents not in the country on Census Night are out of scope of the Census.

Topics collected by the Census change from time to time. There must be a demonstrated national need for Census data for policy development, planning and program monitoring. Details on the changing content of Censuses from 1911 to 2016 can be found on the downloads tab of Census of Population and Housing: Nature and Content, Australia, 2016. A copy of the 2016 Census Household Form is included in the Appendix to the Census Dictionary, 2016 (cat. no. 2901.0).

While the range of topics covered in the 2016 Census was the same as for the 2011 Census, there were some changes in the way some questions were either asked or collected. Examples include: changing the question format for religious affiliation by moving 'No religion' to be the first response category; country of birth of mother and father questions were amended to allow respondents to provide details of the country of birth of their mother and father (where it is not Australia); the introduction of some targeted supplementary questions for occupation and industry on the online form; and changes to the collection of data related to dwellings for mail-out areas in line with establishment of the Address Register. Updated classifications were used for the coding of geographical units, occupation, industry, cultural and ethnic groups, language, religion and countries. For more detail see the 2016 Census Dictionary.

Timeliness

The Census and Statistics Act 1905 requires the Australian Statistician to conduct a Census on a regular basis. Since 1961, a Census has been held every 5 years. The 2016 Census was the 17th national Census, and was held on the 9^th August 2016.

For the 2016 Census there were three main releases:

• An early release of information in April 2017 (8 months after the Census). This release provided a preview of key characteristics of a 'typical' Australian at the national and state/territory level showing what has changed over time.
• The first comprehensive Census dataset was released in June 2017 and provided Community level Census data data for a wide range of topics, including information on small population groups and for small geographic areas such as Suburbs and Local Government Areas.
• Detailed Census data was released in October 2017 and contained data on employment, qualifications and population mobility, including journey to work and internal migration.