Tuesday, 8 August 2006 was Census night in Australia. Every person present in Australia on Census night, excluding foreign diplomats and their families, should have been 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 included more than once (or included when they shouldn't be). In Australia, the Census Post Enumeration Survey (PES) is used to determine how many people were missed in the Census and how many were counted more than once. The PES is a household survey conducted by the Australian Bureau of Statistics (ABS) shortly after the Census. The survey provides an independent check on Census coverage and also identifies the key demographic characteristics of the population that have been missed or overcounted in the Census.
WHAT IS UNDERCOUNT?
In the 2006 Census, some people were missed (undercount) and some were counted more than once (overcount). As is usually the case, in 2006 more people were missed than overcounted.
The PES interview process determines whether each person in the PES sample should have been counted in the Census, and the category in which they should have been counted (such as age, sex, Indigenous status, region of usual residence). The match and search process determines how many times each person in the PES sample was actually counted in the Census, and in which categories.
PES output processing and estimation combines and weights results from the match and search process to produce an estimate of the number of people who should have been counted in the Census. Net undercount is estimated as the difference between this estimate and the actual Census count (including imputed people for non-responding dwellings).
The net undercount for a category of person is obtained by taking the PES estimate of the number of people in the category who should have been counted and subtracting the Census count of the number of people (in the category). Net undercount for a category of person is the net result of undercount, overcount, differences in classification between the PES and Census (e.g. age, sex, Indigenous status), and imputation error in the Census.
Rates of net undercount vary significantly for different population groups depending on factors such as age, sex, ethnicity (including Indigenous status) and geographic location.
PES estimation involves assigning a 'weight' to each selected PES dwelling and then to each person for whom a PES response was obtained.
Dwelling weighting for the 2006 PES comprised two stages. For private dwellings selected in the PES that were found in the Census, the first stage of weighting adjusted the PES selection weight (the inverse of the probability of a dwelling being selected in the PES sample) such that the adjusted weights added up to the Census private dwelling count within categories based on geography and dwelling characteristics. A first-stage weight adjustment was also applied to private dwellings selected in PES that were missed in the Census. For dwellings in discrete Indigenous communities, a similar first-stage weight adjustment was applied based on dwelling counts for communities within each state and territory.
The second stage of dwelling weighting applied a non-response adjustment so that the responding PES dwellings represented other dwellings from which no response was obtained.
The initial stage of person weighting adjusted the dwelling weights to ensure that the PES estimates of people counted (in the Census) in private dwellings and discrete Indigenous community dwellings (other than late-return or non-responding dwellings) in a set of benchmark categories matched the actual Census counts for these categories. The weight adjustment applied to a person did not depend on whether they responded in the Census, but only on characteristics of the person as reported in the PES.
As a final step in weight adjustment, the initial person weights were adjusted so that the PES estimates also represented people in non-private dwellings, such as hotels, hospitals and jails, which were not covered by the PES. The person weighting step in PES processing calculates weights for all PES records, including those relating to late-return or non-responding dwellings.
The PES estimate of the number of people who should have been counted in the 2006 Census was 20,402,459 people. The actual 2006 Census count for Australia was 19,852,973 people. The difference (549,486 people) is the net undercount for Australia.
Further information on Census late-return or non-responding dwellings can be found in the Appendix of this publication.
USE OF NET UNDERCOUNT IN ESTIMATED RESIDENT POPULATION (ERP)
After each Census, the ABS uses the new information obtained to rebase the estimated resident population (ERP) of Australia and the states and territories. A crucial step in rebasing ERP using the 2006 Census counts is the application of the net undercount estimates from the PES. For more information on the calculation of rebased ERP for 30 June 2006 based on results from the 2006 Census and PES, see Australian Demographic Statistics, December quarter 2006 (cat. no. 3101.0), released on 5 June 2007.