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THE REBASING PROCESS
The ABS takes a Census of Population and Housing every five years as required by the Census and Statistics Act 1905. Regularly taking a Census enables the updating and maintenance of an accurate time series of Australia's official population estimates. This updating process is referred to as 'rebasing' and allows the ABS to revise official population estimates for the quarters back to the previous Census. Rebasing maintains and improves the accuracy of the population series by limiting the accumulation of errors to a five year period.
ESTIMATED RESIDENT POPULATION
ERP is always based on the most recent Census data. It is maintained in between Censuses through an accounting framework that updates the base or 'stock' population with data on the 'flow' of demographic events. These demographic events are called the components of population change and consist of births, deaths and migration. Data for components of population change come from three main sources; birth registrations, death registrations and overseas arrivals and departures information converted into net overseas migration.
The accounting framework is represented by the demographic balancing equation:
represents the base resident population
represents births over the period
represents deaths over the period
represents net migration over the period
represents the intercensal error term
Rebasing the ERP produces a new term every five years. The ERP as at 30 June in a Census year (for example 2001, 2006, 2011, etc) is calculated using newly available Census data together with an estimate of Census undercount, residents temporarily overseas (RTOs) on Census night, and an adjustment to backdate from Census night to 30 June.
Intercensal error is an all-encompassing term reflecting error in all of the components of population change (births, deaths and net overseas migration) and error in either or both of the bases. The accumulation of error between rebasing periods can only be measured after a Census.
OVERVIEW OF THE REBASING PROCESS
There are two main stages to the rebasing process. The first stage is to create the new base, known as ERP as at 30 June of the Census year. The second stage is to use this new base to revise the ERP series between the new Census year and the Census year five years prior.
The process of creating the new 30 June ERP base is shown in Figure 1 below:
The next process is to create an estimate of the resident population as at Census night. This is done by applying an estimate of people who were not counted in the Census, applying demographic adjustments, and adding in residents temporarily overseas.
The last process undertaken to create Australia's official population estimate, ERP, as at 30 June is to take the estimate of the resident population for Census night and backdate it to 30 June. This is done by removing births and adding in any deaths that occurred between 30 June and Census night. The total is also adjusted by the net interstate migration and net overseas migration that took place between 30 June and Census night.
The process undertaken in 2011 is outlined in further detail in Information Paper: Rebasing Population Estimates, Australia, 2011 (cat. no. 3101.0.55.001), published by the ABS on 22 September 2011.
PRELIMINARY AND FINAL ESTIMATES
The ABS releases the new official population base after each Census as both a preliminary and final estimate. Preliminary ERPs are released as soon as possible after the Census but before the 30 June of the year following the Census. This is to meet legislative reporting requirements. The Preliminary ERPs are changed to Final ERPs twelve months later when final net overseas migration data becomes available for use in both the RTO calculation and for the backdating process. This time period also allows the possibility of revision to births and deaths data due to processing of late birth and death registrations.
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