THE REBASING PROCESS

BACKGROUND

The official estimate of Australia's population is known as Estimated Resident Population, or ERP. It is published by the Australian Bureau of Statistics each quarter.

ERP statistics are used to inform evidence-based decisions such as:

• allocating Goods and Services Tax (GST) revenue and other funds to the states and territories as set out in the Federal Financial Relations Act 2009 and the Local Government (Financial Assistance) Act 1995; and
• allocating the number of seats for the House of Representatives between the states and territories as set out in the Commonwealth Electoral Act 1918 to ensure representation that is proportional to population.

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:

Where:
represents the population being estimated,
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:

Figure 1: The Process of Creating the New ERP Base 
 

Figure 1 shows Census counts are first adjusted to estimate the population on a usual residence basis. This is done by statistically moving people back home. This means both assigning people to the state or territory in which they usually live and removing the Census count of visitors from overseas.

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.