FEATURE ARTICLE: PRELIMINARY REBASING OF AUSTRALIA'S POPULATION ESTIMATES USING THE 2006 CENSUS OF POPULATION AND HOUSING
The Feature Article Preliminary rebasing of Australia's population estimates using the 2006 Census of Population and Housing has been reissued to correct errors in the subtotal and total calculations in the table Adjustment components of estimated resident population, preliminary - 30 June 2006.
In this issue, the ABS has used the 2006 Census results to make two main series of calculations to produce preliminary rebased population estimates. These estimates will be updated again to produce final rebased estimates in the December quarter 2007 issue of this publication (released in June 2008).
The first series of calculations was applied to construct a new preliminary estimated resident population (ERP) for 30 June 2006 from which to then estimate quarterly ERP forward. This was done to ensure that population estimates for the next intercensal period (i.e. 2006-2011) are as accurate as possible. Because this new population estimate uses the Census as its main data source, it is said to be 'based' on the 2006 Census and is referred to as a population base.
The second series of calculations was made to revise the 19 intercensal quarterly estimates preceding June quarter 2006 (i.e. September quarter 2001-March quarter 2006) to produce 'preliminary rebased' estimates. When the status of these estimates is changed to 'final rebased' in the December 2007 issue of this publication to be released in June 2008, no subsequent revisions will be made to these estimates. Making this adjustment ensures that the ERP time series for the previous intercensal period are comparable with the latest estimates. The following provides a more detailed explanation of the adjustments made using the 2006 Census results and some of the related rebasing concepts.
CONSTRUCTING THE PRELIMINARY ERP BASE FIGURE FOR 30 JUNE 2006
Constructing a preliminary 30 June 2006 ERP figure from the 2006 Census count involved two main steps. The first step was to calculate ERP for Census night 8 August 2006. This involved:
The table below shows how the ABS arrived at the Census night 8 August 2006 ERP figure for Australia, states and territories using figures obtained from applying the adjustments listed above.
- retabulating the Census counts by actual location to reflect a Census count by place of usual residence by:
- adding in an estimate of those residents who were absent interstate on Census night;
- subtracting those who were visiting from Interstate or were overseas visitors to Australia on Census night;
- accounting for net undercount using the results from the Post Enumeration Survey (PES);
- adding in the number of Australian residents who were temporarily overseas (RTOs) on Census night using data on international travellers obtained from the Department of Immigration and Citizenship; and
- applying a range of demographic adjustments designed to resolve any statistical anomalies in the age sex composition of the derived population estimates.
Adjustment components of estimated resident population, preliminary - 30 June 2006
|Components as at 8 August 2006: |
|Census count, actual location |
|plus - Residents absent interstate |
|less - Interstate visitors |
|less - Overseas visitors |
|equals - Census count, place of usual residence |
|plus - Allowance for under-enumeration(b) |
|plus - Demographic adjustment(b) |
|plus - Residents temporarily overseas(b)|
|equals - ERP as at 8 August 2006(b) |
|Backdating components to 30 June 2006: |
|less - Births(b)(c) |
|plus - Deaths(b)(c) |
|less - Net interstate migration(b)(c) |
|less - Net overseas migration(b)(c) |
|equals - preliminary ERP 30 June 2006 |
|. . not applicable |
|- nil or rounded to zero (including null cells) |
|(a) Includes Other Territories - see paragraph 2 of the Explanatory Notes. |
|(b) These estimates are preliminary and may change in compilation of final rebased estimates. |
|(c) Component data calculated for the period 1 July to 8 August 2006. |
The second main step in arriving at ERP for 30 June 2006 was to back-date the resulting ERP figure for Census night 8 August 2006 to 30 June 2006 using the components of population change. This involved:
The table above shows how the ABS arrived at the preliminary rebased ERP figure for Australia, its states and territories at 30 June 2006 using births, deaths and migration figures.
- subtracting births;
- adding in deaths; and
- subtracting net interstate migration and net overseas migration.
INTERCENSAL ERROR AND INTERCENSAL DISCREPANCY
The intercensal error refers to the difference between the latest Census based ERP figures for 30 June 2006 and the ERP figures based on the previous Census which have been carried forward using births, deaths and migration data.
There are two areas that contribute to the intercensal error:
The table below shows the preliminary intercensal error by number and percentage of total population for the 2001-2006 period for Australia, states and territories.
- errors in the Census based estimates of the population at the current or previous Census date; and/or
- errors in the estimates of any of the components of population change (births, deaths and migration) since the previous Census.
Intercensal Error(a), 2001-2006
|New South Wales |
|South Australia |
|Western Australia |
|Northern Territory |
|Australian Capital Territory |
|(a) A positive number indicates that unrebased ERP as at 30 June 2006 was higher than rebased ERP. A negative number indicates it was lower than rebased ERP. |
|(b) Includes Other Territories - see Paragraph 2 of the Explanatory Notes. |
Information collected in the 2006 Census will also allow the ABS to estimate approximately how much of the intercensal error is due to inaccuracies in estimates of interstate migration. In order to do this, the ABS will analyse data from the Census questions concerning an individual's place of usual residence one year ago and five years ago. These results will be released in the December 2007 issue of this publication (released in June 2008).
After the intercensal error is adjusted for revisions to the components of population change (births, deaths and migration), the remaining (unattributable) portion is referred to as the intercensal discrepancy. The estimate of intercensal discrepancy for each state and territory, birth cohort and sex are spread evenly across the intercensal quarters. Thus the intercensal discrepancy acts as a balancing item, that when combined with births, deaths and migration equals the difference between the two Census population estimates. Intercensal discrepancy is caused by errors in the start and/or finish population estimates and/or in estimates of births, deaths or migration in the intervening period which cannot be attributed to a particular source. The 2001-2006 intercensal discrepancy will be published in the December 2007 issue of this publication, to be released in June 2008.
REVISING THE 20 MOST RECENT QUARTERLY ESTIMATES TO 'PRELIMINARY REBASED'
The main purpose of revising the 20 most recent intercensal estimates (i.e. September 2001-June 2006) of quarterly population growth to 'preliminary rebased' was to ensure that the estimates from the 2001-2006 intercensal period will be comparable to all future estimates, thus creating a consistent time series of ERP data. These estimates will remain as 'preliminary rebased' until the December 2007 issue of this publication (released in June 2008) when their status will be changed for the last time to 'final rebased'. Following this, no subsequent revisions will be made to these estimates.
ADJUSTING FOR NET UNDERCOUNT
Net undercount for Australia in the 2006 Census was 549,600 persons. Net undercount is the difference between the actual Census count and the estimate of the number of people who should have been counted in the Census. This estimate is based on the PES conducted in August and September of 2006. For a category of person (based on age, sex and state of usual residence), net undercount is the resultant of Census undercount, overcount, misclassification and imputation error. Adding the net undercount of people back into the population is a crucial step in arriving at the most accurate ERP possible. For more information on measuring net undercount using the PES see Information Paper: Measuring Net Undercount in the 2006 Population Census, 2007 (cat. no. 2940.0.55.001) and Census of Population and Housing - Undercount (cat. no. 2940.0).
CHANGES MADE DURING THE 2006 CYCLE
Expanding the coverage of PES
The ABS has improved the PES results by expanding its coverage to include both remote areas of Australia and discrete Indigenous communities. This expansion was undertaken to ensure that persons living in these areas and communities had an equal chance of selection for the survey. Previously, when PES did not cover these areas and communities, the ABS made an assumption that undercount in these areas were represented by survey responses for the rest of each state and territory.
New method for defining Residents Temporarily Overseas
ABS has improved the measure of net overseas migration by expanding the Australian residence criteria from 12/12 months to 12/16 months (see Information Paper: Improved Methods for estimating Net Overseas Migration (cat. no. 3107.0.55.003)). This had implications for the measurement of residents temporarily overseas (RTOs) which are included in preliminary rebased ERP. Of the Australian residents absent on Census night who return within 12 months, the great majority return within five months. The ABS now uses resident status propensities based on recently observed (2005) traveller behaviour to determine the number of RTOs by inflating the observed number to represent the full 12 month period directly following the Census.
PLANS FOR FURTHER OUTPUT
Following this issue which contains preliminary population estimates for Australia, states and territories based on the 2006 Census, the ABS will also publish final population estimates based on the 2006 Census for Australia, states and territories, for September quarter 2001 to June quarter 2006, in the December quarter 2007 issue of Australian Demographic Statistics (cat. no. 3101.0) to be released on 5 June 2008. That issue will include final rebased estimates dating back to 30 September 2001. No subsequent revisions will be made to those final rebased estimates.
Preliminary rebased estimates for SLAs and LGAs will be published in late July 2007 in Regional Population growth, Australia, 1996-2006 (cat. no. 3218.0) with final rebased estimates published in July/August 2008.
Preliminary rebased estimates of the Australian Aboriginal and Torres Strait Islander population at 30 June 2006 will be published in Population Distribution Aboriginal and Torres Strait Islander Australians, 2006 (cat. no. 4705.0) in August 2007. Final rebased estimates for 30 June 2006 will be published in July/August 2008.