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Some of the reasons why people may have been counted more than once (i.e. overcounted) or in error include:
WHAT IS NET UNDERCOUNT?
While every effort is made to eliminate these potential causes of error, some undercount and overcount will inevitably occur. As is usually the case in Australia, in the 2011 Census more people were missed than overcounted and so the Census count of the population is fewer than the true population. This difference is referred to as net undercount.
Net undercount for any category of person is the difference between the PES estimate of the number of people who should have been counted in the Census and the actual Census count (including imputed persons in non-responding Census dwellings).
KEY USES OF NET UNDERCOUNT
Net undercount is the primary measure of Census coverage, and as such, is used in the following ways:
Accurate estimates of the resident population are required for a wide range of uses including: the allocation of seats to states and territories in the House of Representatives of the Australian Parliament; the distribution of Commonwealth payments to states and territories; as well as demographic, social and economic studies.
For more information on the calculation of ERP for 30 June 2011 based on results from the 2011 Census and PES, see the ABS publication Australian Demographic Statistics, December quarter 2011 (cat. no. 3101.0), released on 20 June 2012.
THE IMPORTANCE OF EFFECTIVE STATISTICAL INDEPENDENCE
The PES is designed to be an independent check of Census coverage. Therefore, it is critical that the statistical independence between the PES and the Census is effectively managed, to ensure the PES is a robust check on Census coverage.
There are two aspects to statistical independence, both of which were effectively managed throughout the 2011 PES cycle: population independence and operational independence.
Population independence refers to the principle that there should be no sub-groups of the population where being missed in the Census indicates that a person or dwelling is also more likely to be missed by the PES. Although the PES estimation process adjusts for this to some extent, by subdividing the population into smaller groups where the assumption of population independence is more likely to be true, population independence is always more difficult to achieve than operational independence.
Selection to participate in the 2011 PES was based on a sample of private dwellings, meaning that those persons who were not living in, or visiting, a private dwelling at PES time were unavailable for selection. Therefore, although the PES has shown it is very effective in assessing overall Census coverage, its usefulness for estimating the undercount of certain sub-populations is limited, such as fly-in fly-out workers, who often live in non-private dwellings.
Operational independence requires that Census operations do not influence the PES in any way, and vice versa. The operational independence of the PES from the Census was effectively monitored at every stage of the 2011 cycle, including enumeration, processing and administration. Steps taken to ensure this independence included:
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