Small area population projections
Updated: 7 December 2007
An ABS Demography Consultancy Project
This report outlines the method used for producing population and enrolment projections for all Census Collection Districts (CCDs) in Western Australia, spanning from 1999 to 2004.
The technique employed for the projections was the cohort-component method, widely accepted as the best way of producing age/sex population projections. It involved applying fertility, mortality and interstate migration rates and overseas migration levels by age and sex to the base population to produce a projected population, which then became the base for projecting the next year. This cycle was repeated until the projection horizon was reached.
A three-tiered approach was taken in projecting resident population aged 18 years and over for all Statistical Local Areas (SLAs) and CCDs in Western Australia.
- The Western Australian population was projected by age and sex.
- The population of all Western Australian SLAs was projected by age and sex (constrained to 1).
- The SLA projections were split into CCDs.
Finally, the projections were grouped into persons aged 18 years and over, and aligned with enrolment data to produce projected enrolments.
- State Projections
The base population for the Western Australian cohort-component projections was preliminary age/sex Estimated Resident Population (ERP) as at 30 June 1999. Assumptions for the projections were based on both short and long-term trends for each component of population change. The fertility (low : Aust 2006-51 TFR=1.60) and overseas migration (high : Aust Net=90 000) assumptions were from the latest Population Projections of Australia, States and Territories 1997–2051 (ABS Cat. 3222.0), although the mortality assumptions were adjusted to reflect mortality levels evident since 1997. Interstate migration assumptions were interpolated between actual 1998–99 figures and assumed (medium scenario) levels for 2003-04 from Population Projections.
- SLA Projections
The base population for the SLA cohort-component projections was preliminary 30 June 1997 SLA age/sex ERP. The fertility, mortality and migration assumptions were based on an assessment of SLA-specific levels and trends observed since 1991, with the net migration assumption adjusted to take into account the most recent state government population projections and/or land planning information. The ABS regularly collects demographic information down to the SLA level, which means that SLA projections (in contrast to smaller areas) are firmly based on a series of known data. At each yearly cycle in this process, the SLA projections were constrained to sum to the State projection, helping to produce more reliable SLA results. Most SLAs with ERP under 1000 persons were held constant for the projection duration as assumptions for the accompanying tiny age/sex cells are too unreliable.
Although the base population used was at June 1997, actual SLA net migration was known for the two following financial years, so these were used as the assumptions up to June 1999. The 2003–04 SLA net migration assumptions were the long-term levels used in the projections ABS has placed on the Integrated Regional Database (ABS Cat. 1353.0), and reflect historical trends in SLA migration, State Government projections and capital city/balance of State levels used in Population Projections. Migration assumptions for 1999–2000 to 2002–03 were derived by interpolation, providing a transition between short and long-term levels.
- CCD Projections
The basis for calculating CCD projections was an SLA to CCD concordance derived from the 1996 Census of Population and Housing. Differing growth rates of CCDs within SLAs were incorporated using 1991–1999 ABS CCD building approval data, resulting in SLA to CCD concordance split factors extrapolated through to the year 2004. These were applied to the SLA projections to give CCD projections, then adjusted to reflect projected enrolments as at 31 March 2004.
The lack of demographic data collected regularly at CCD level makes it necessary to use such a conversion method as outlined above. While the process is quite complex, it should be reiterated that the basic concept of splitting SLAs to CCD level cannot be expected to give projections as reliable as those for SLAs. However, as the end product will be aggregates of large numbers of CCDs there is a high likelihood that any random errors or inconsistencies will be statistically offset in the aggregation process.
The SLA projections are on the most recent boundaries available, that is, those on which 30 June 1997 SLA ERPs have been be published. This corresponds to the Australian Standard Geographical Classification (ASGC), 1996 Edition (ABS Cat. 1216.0). All CCDs are on 1996 Census boundaries.
It is important to recognise that the projection results given in this report essentially reflect the assumptions made about future fertility, mortality and migration trends. While these assumptions are formulated on the basis of an objective assessment of demographic trends over the past decade and their likely future dynamics, there can be no certainty that they will be realised.
ABS takes responsibility for the method employed, however in accordance with ABS policy regarding small area population projections, the assumptions used are the final responsibility of the client, and the projections are not official ABS population statistics.
The projections may be referred to as "…projections prepared by the ABS according to assumptions reflecting prevailing trends agreed to by the Australian Electoral Commission…".
No liability will be accepted by the ABS for any damages arising from decisions or actions based upon this population projection consultancy service.
Australian Bureau of Statistics
Note: A more detailed explanation of the methods used by ABS to produce the enrolment projections shown in this volume is available from:
- The Australian Electoral Officer for WA, Burswood Tel: (08) 9470 7299
- The Federal Redistribution Secretariat, Canberra Tel: (02) 6271 4688