Why are people missed in the Census?

The Census is a large and complex exercise. As such, there are always going to be a small number of people that are missed and an even smaller number who are counted more than once.

People may be missed due to a number of reasons. For instance, they may have been travelling and were difficult to contact, the person completing the form may have mistakenly thought that they shouldn't be counted, or the dwelling was difficult to find.

The Post Enumeration Survey

The Post Enumeration Survey (PES) is a short survey run in the month after each Census, to determine how many people were missed or counted more than once.

It is run independently from the Census, to provide an independent assessment of the completeness of the Census.

From the survey it is possible to determine the number of people who were counted in the Census and the number of people who should have been counted in the Census. The difference is referred to as net undercount.

Net undercount in 2011

The 2011 PES found the net undercount for the 2011 Census was 375,000 people, or 1.7% of the population. This means the Census managed to count over 98% of all people in Australia on Census night, which was a great result and emphasised the continuing cooperation of Australians in this important event.

In contrast, in 2006 the net undercount was 550,000 people, or 2.7%.

How net undercount is used

Net undercount has three important purposes:
  • Obtaining an accurate estimate of Australia's population (by adjusting for people missed in the Census)
  • Providing a measure of the completeness of the Census, to assist in interpreting Census data
  • Identifying improvements for future Censuses.

Accurate population estimates are important for Australia as they are used to distribute federal funds to states and territories, and determine where services such as schools and hospitals are needed.

The reliability of PES results

More than 40,000 households were selected to participate in the 2011 PES, which makes the PES one of the largest household surveys conducted by the ABS.

A random sample of households was selected, designed to provide a balanced picture of Australia. From this sample, the ABS collected information from over 97,000 people.

Most people in the selected households responded to the PES willingly. The high ‘response rate’ for the survey also ensures that the PES results provide a balanced picture of Australia, with survey errors minimised.

Information collected in the PES

The PES is designed to collect just enough information to determine whether someone was counted in the Census and to enable the ABS to produce a range of net undercount information.

This included important demographic information (name, sex, age, date of birth, country of birth), the address where each person was on Census night, and any other addresses where each person may have been included on a Census form.

While the PES asked whether each respondent thought they were included on a Census form, the ABS actually compared Census and PES data to determine whether someone was in fact counted or not.

For more information

Further information on the PES and net undercount in the 2011 Census can be found in Census of Population and Housing - Undercount, Australia, 2011 (cat.no. 2940.0).