**ESTIMATED RESIDENT POPULATION (ERP)**

The Australian Bureau of Statistics (ABS) prepares and publishes official estimates of the population, according to the concept of estimated resident population (ERP), which links people to a place of usual residence within Australia. Usual residence is that place where each person has lived or intends to live for six months or more from the reference date for data collection. Estimates of the resident population are based on counts from the five-yearly Census of Population and Housing ('Census') by place of usual residence (excluding overseas visitors in Australia), with an allowance for net undercount in the Census, to which are added the number of Australian residents estimated to have been temporarily overseas at the time of the Census.

Population estimates for Australia and the states and territories are updated quarterly by adding to the estimated population, at the beginning of each period, the components of natural increase (births minus deaths, on a usual residence basis) and net overseas migration. For the states and territories, account is also taken of estimated interstate movements involving a change of usual residence.

Below the state and territory level, the absence of recent regional migration data means a different approach is taken to estimate population change since the Census. Instead of taking into account the components of population growth, a mathematical regression model is used to estimate the combined effect of natural increase and net migration for each region's total population. The total population for each region is then broken down into its estimated age and sex components. Sub-state population estimates are prepared and released annually, as at 30 June each year.

Under the ASGC, the Statistical Local Area (SLA) is the base spatial unit used to prepare and disseminate sub-state population estimates. The SLA is the smallest unit defined in the ASGC in a non-Census year. All estimates at the SLA level are constrained to sum to state and territory population estimates.

When population estimates as at 30 June in a Census year are prepared, based on that year's Census, they can be compared with the preliminary estimates for the same date already produced by updating the previous Census. Differences between these two sets of estimates are known as intercensal errors. Estimates for the preceding intercensal period are updated by apportioning the intercensal error evenly across the five years. These estimates are known as 'rebased' estimates.

To meet the conflicting demands for accuracy and timeliness, there are several versions of sub-state/territory population estimates. Preliminary estimates as at 30 June are normally available by April the following year, revised estimates twelve months later and rebased and final estimates after the following Census.

Population estimates are used for a wide variety of purposes, including:

- the distribution of Australian Government funds to state, territory and local governments;
- the apportionment of the number of seats in the House of Representatives;
- the formulation, monitoring and evaluation of government policy;
- market research;
- academic research; and
- the denominator in per capita rates, fertility and mortality rates.

More detailed explanations of the concept of ERP, as adopted by the ABS for official population estimates, are contained in

__Information Paper: Population Concepts, 2008__ (cat. no. 3107.0.55.006) and

__Population Estimates: Concepts, Sources and Methods__ (cat. no. 3228.0.55.001).