2940.0.55.001 - Information Paper: Measuring Net Undercount in the 2011 Population Census, 2011  
ARCHIVED ISSUE Released at 11:30 AM (CANBERRA TIME) 06/07/2011   
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The Population Census is a valuable data source for estimating the size and geographic distribution of the Australian population, and for analysing the major demographic, social and economic characteristics of the population, particularly for small geographic regions and other small sub-populations. It provides statistics for decision-making by governments, businesses, community organisations and individuals. A Census also provides a base for post-censal population estimates and projections, which assist in planning and policy-making at the national, state and local levels.

A Census is conducted in Australia every five years. The next Census will be held on the night of Tuesday 9 August 2011. On Census night, every person present in Australia, excluding foreign diplomats and their families, should be included on a Census form at the place where they stayed.


Whenever a Census is undertaken, questions about the completeness and accuracy of the Census count invariably arise. In such a large and complex exercise, it is inevitable that some people will be missed and some will be counted more than once. Some of the reasons why people may be missed (i.e. undercounted) include:

    • they were travelling and were difficult to contact;
    • they mistakenly thought they were counted elsewhere;
    • there was insufficient space on the Census form in the household where they were staying and they did not obtain additional forms;
    • the person completing the form thought that, for example, young babies, the elderly or visitors should not be included;
    • they did not wish to be included due to concerns about the confidentiality of information or a more general reluctance to participate;
    • the dwelling in which they were located was missed because it was difficult to find (e.g. in a remote or non-residential area); and
    • the dwelling in which they were located was mistakenly classed as unoccupied.
Some of the reasons why people are counted more than once (i.e. overcounted) include:
    • they were included on the Census form at the dwelling where they usually live, even though they stayed and were counted elsewhere on Census night; and
    • they were overseas on Census night and so should not have been counted at all, but were included on the Census form at the dwelling where they usually live.
While every effort is made to eliminate these potential causes of error, some undercount and overcount will occur. Usually more people are missed than overcounted in Australia, so the Census count of the population is fewer than the true population. This difference is called net undercount. Rates of net undercount can vary significantly for different population groups depending on factors such as sex, age, ethnicity (including Indigenous origin) and geographic location. The ABS obtains estimates of net undercount using information collected in a post-enumeration survey conducted immediately following the Census.

Estimates of net undercount are used to:

    • derive an estimate of the resident population for 30 June of the Census year;
    • provide users with an assessment of the completeness of Census counts, allowing them to take this into account when using Census information; and
    • evaluate the effectiveness of Census collection procedures so that improvements can be made for future Censuses.
Accurate resident population estimates are required for a wide range of uses, including the allocation to states and territories of seats in the Federal House of Representatives; the distribution of Commonwealth payments to states and territories; and demographic, social and economic studies.


The purpose of this paper is to set out the methods to be used in the conduct of the 2011 Census Post Enumeration Survey.