This report outlines the method used for producing population and enrolment projections for all Census Collection Districts (CDs) in the Australian Capital Territory, spanning from 2004 to end April 2009.
The main 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 annual fertility and mortality rates and internal migration and overseas migration 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 CDs in the Australian Capital Territory.
Finally, the projections were grouped into persons aged 18 years and over, and aligned with enrolment data to produce projected enrolments.
The base population for the Australian Capital Territory cohort-component projections was revised age/sex Estimated Resident Population (ERP) as at 30 June 2003. Assumptions for the projections were based on both short and long-term trends for each component of population change. The fertility (medium: Aust. 2011-51 TFR=1.60), mortality (medium scenario), overseas migration (medium: Aust. Net=100,000) and interstate migration assumptions were from Series B in the latest Population Projections of Australia, States and Territories 2002–2101 (ABS Cat. 3222.0). The interstate migration assumption for the ACT was amended to -1,500 for 2003/4 to reflect current trends and thereafter remained constant at -500 as per Series B.
The base population for the SLA cohort-component projections was revised 30 June 2003 SLA age/sex ERP. The fertility, mortality and migration assumptions were based on mean SLA-specific levels observed over the last five years, constrained to the assumed Territory levels and trends. The net migration assumptions also incorporated dwelling forecast data from ACT Government and included some adjustments to prevent any SLA declining too rapidly. SLA age/sex migration profiles were derived from 2001 Census data on place of usual residence one year ago.
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 Territory projection, helping to produce more reliable SLA results. Most SLAs with ERP under 500 persons were held constant for the projection duration as assumptions for the accompanying tiny age/sex cells are too unreliable.
CD projections were formed using extrapolations from 2001–2003 CD ERP constrained to the SLA projections. CD ERP is derived using 2001 Census CD-to-SLA usual residence population proportions updated for post-censal growth using CD building approvals. This approach allows for sub-SLA differential growth while retaining consistency with the SLA projections. The final process adjusts the CD projections for persons aged 18 and over to reflect projected enrolments as at 30 April 2009 using the November 2004 relationship between each CD's enrolments and ERP.
The lack of demographic data collected regularly at CD 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 CD 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 CDs there is a high likelihood that any random errors or inconsistencies will be statistically offset in the aggregation process.
CD boundaries are from the Australian Standard Geographical Classification (ASGC), 2001 Edition (ABS Cat. 1216.0), corresponding to those used for the 2001 Census. SLA boundaries are from a subsequent ASGC version, the 2003 Edition.
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 historical demographic trends 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.