2940.0.55.002 - Information Paper: Measuring Overcount and Undercount in the 2016 Population Census, Jul 2016  
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INTRODUCTION

THE CENSUS OF POPULATION AND HOUSING

The Census is a valuable data source for estimating the size and geographic distribution of Australia's population, and for analysing the major demographic, social and economic characteristics by governments, businesses, community organisations and individuals. It is particularly useful for examining small geographic regions and small population groups. The 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 level.

A Census is conducted in Australia every five years. The next Census will be held on the night of Tuesday 9 August 2016. 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.


OVERCOUNT AND UNDERCOUNT


Whenever a Census is undertaken, questions about the completeness and accuracy of the Census count invariably arise. With such a large and complex exercise, it is inevitable that some people will be counted more than once and some may be missed.

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.

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 dwelling where they were staying and additional forms were not obtained;
  • 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 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.

After the Census collection period is finished, there are usually a small proportion of dwellings for which no Census form has been submitted. If Census fieldwork determines these dwellings are occupied, field information is used to impute a plausible number of persons present on Census night. As this imputation is approximate, it can also contribute to overcount or undercount.


POST ENUMERATION SURVEY

While every effort is made to eliminate these potential causes of error, some overcount and undercount will occur. The ABS obtains estimates of overcount and undercount using information collected in the Post Enumeration Survey (PES), which is conducted immediately following the Census.

If the PES finds that the total number of persons counted more than once in the Census is greater than the number of persons missed, the difference is called a net overcount. If the PES finds that the number of persons missed by the Census is greater than the number who were counted more than once, the difference is called a net undercount. Rates of net overcount or net undercount can vary significantly for different population groups depending on factors such as sex, age, ethnicity (including Indigenous origin) and geographic location.

Estimates of overcount and 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; planning and reporting on performance; and demographic, social and economic studies.


PURPOSE OF THIS PAPER

The purpose of this paper is to set out the methods to be used in the conduct of the 2016 Census Post Enumeration Survey.