Small area population projections

Updated: 7 December 2007

An ABS Demography Consultancy Project

Victorian Small Area Populations (ABS)

This report outlines the method used for producing population and enrolment projections for all Census Collection Districts (CCDs) in Victoria, spanning from 2001 to 2006.

Projection Method

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 four-tiered approach was taken in projecting resident population aged 18 years and over for all Statistical Local Areas (SLAs) and CCDs in Victoria.

  1. The Victorian population was projected by age and sex.
  2. The population for Melbourne and balance of Victoria was projected by age and sex and constrained to the population in 1.
  3. The population of all Victorian SLAs was projected by age and sex and constrained to the appropriate population in 2.
  4. The SLA projections were split into CCD projections.

Finally, the projections were grouped into persons aged 18 years and over, and aligned with enrolment data to produce projected enrolments.

1. State Projections

The base population for the Victorian cohort-component projections was the preliminary age/sex Estimated Resident Population (ERP) at 30 June 1999. Assumptions for the projections are the same as those used for Victoria in Series II outlined in Population Projections, Australia 1999–2101 (ABS Cat. No. 3222.0) published on 17 August 2000. Series II incorporates assumptions for Australia of 'low' fertility (a total fertility rate of 1.60 births per woman from 2008), 'medium' overseas migration (an annual net overseas migration gain of 90 000) and 'medium' interstate migration. The standard mortality assumption results in life expectancy at birth of 83.3 years for males and 86.6 years for females at the Australian level by 2051.

2. Part of State Projections

The base population for the Melbourne and balance of Victoria projections was the preliminary estimated resident population in each area, at 30 June 1999. The assumptions for fertility, mortality, overseas migration and interstate migration are the same as those used for those areas for Series II outlined in Population Projections, Australia 1999–2101 (Cat. No. 3222.0). These part of State projections were constrained to the Victorian projections.

3. SLA Projections

The base population for the SLAs in Victoria was the preliminary estimated resident population in each area by single year of age and sex, at 30 June 1999.

The fertility assumptions were based on average age-specific fertility rates observed in each SLA between 1991 and 1997, projected to change in line with the Victorian assumptions outlined in Population Projections, 1997 to 2051 (ABS Cat. No. 3222.0) published on 14 July 1998. The mortality assumptions were based on the Victorian age-and-sex-specific mortality rates. Assumptions for 1998–2006 were calculated using the rates of mortality decline observed in Victoria between 1987–91 and 1992–96. Assumptions beyond 2006 were calculated using the rates of mortality decline observed in Australia during 1967–71 to 1992–96. The mortality assumption was then adjusted to reflect the recent mortality experienced in each SLA.

In constraining the SLA population projections to the projections published in Population Projections, Australia 1999–2101, the fertility and mortality assumptions used in the latter projections will in effect filter down to the SLA projections.

The assumed migration levels were based on historical trends of net migration in each SLA, the assumed levels of Victorian overseas and interstate migration and any recent Victorian government dwelling and population projections or local land planning information. The assumed SLA net migration levels were constrained to the Melbourne and balance of Victoria assumptions in Series II. The age-sex distributions for the assumed migration levels were based on overseas and inter-SLA migration rates used in the calculation of published ABS SLA age-sex population estimates, which were originally derived from 1996 Census of Population and Housing migration data.

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 Melbourne and balance of Victoria projections as appropriate, helping to produce more reliable SLA results. SLAs with a base population of less than 1 000 were generally kept constant as the age-sex cells are too small for reliable projection.

4. 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 1991–2000 ABS CCD building approval data, resulting in SLA to CCD concordance split factors extrapolated through the projection period. These were applied to the SLA projections to give CCD projections, then adjusted to reflect projected enrolments at 31 March 2006.

The lack of demographic data collected regularly at the 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 the 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.

5. Boundaries

The SLA projections are based on boundaries that existed at the 1996 Census of Population and Housing. This corresponds to the Australian Standard Geographical Classification (ASGC), 1996 Edition (ABS Cat. 1216.0). All CCDs are on 1996 Census boundaries.

6. Disclaimer

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.

The 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.

Demography Section
Australian Bureau of Statistics
January 2002