Text provided by the Australian Bureau of Statistics.
This outlines the process used for producing population and enrolment projections for all Statistical Area 1s (SA1s) in Tasmania, from June 2015 to June 2021.
The method employed for projecting the population down to Statistical Area 2 (SA2) level 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 population for projecting the next year and so on. This cycle was repeated until the projection horizon was reached.
The following four-tiered process was taken in projecting the resident population aged 18 years and over for all SA1s in Tasmania:
Finally, the SA1 projected population aged 18 and over was combined with enrolment data to produce projected enrolments.
The base population for the Tasmania cohort-component projections was preliminary age/sex Estimated Resident Population (ERP) as at 30 June 2015,. Assumptions for the projections were based on both short and long-term trends for each component of population change. These fertility, mortality, overseas migration and interstate migration assumptions were based on those used in the latest Population Projections, Australia, 2012 (base) -2101 (ABS Cat. No. 3222.0), but adjusted to reflect more recently available data. All States and Territories were independently projected, then constrained to sum to the Australian-level projection.
As per the State/Territory level, the capital city and rest of state projections used assumptions updated from the Population Projections publication. 30 June 2015 ERP base population was used, with assumptions reflecting historically observed region-specific patterns of fertility, mortality, overseas migration and internal migration. The Tasmania projections acted as control totals.
The base population for the SA2 cohort-component projections was also 30 June 2015 SA2 age/sex ERP. The fertility, mortality and migration assumptions were based on SA2-specific levels observed during the past five years, constrained to the assumed capital city/rest of state levels and trends. SA2 age/sex migration profiles were derived from 2011 Census data on place of usual residence one year ago, with migration levels based on recent growth rate and proportion of Capital City/Balance of State migration.
The ABS regularly collects demographic information down to the SA2 level, which means that SA2 projections (in contrast to smaller areas) are firmly based on a series of known data. At each yearly cycle in this process, the resulting SA2 projections were constrained to sum to the capital city/rest of state projections, helping to produce more reliable SA2 figures. SA2s with an ERP of less than 1,000 persons were generally held constant for the projection duration as assumptions for the accompanying tiny age/sex cells are too unreliable.
From the resultant 30 June SA2 projections, the projected population aged 18 and over was derived by sub setting the total population for each SA2.
SA1 projected population aged 18 and over were calculated by extrapolation using 2011-2015 SA1 ERP.
SA1 projections were formed using extrapolations from 2011-2015 SA1 ERP constrained to the SA2 projections. Projected population aged 18 at 30 March 2021 was derived by interpolation using the 30 June projections.
Following the above four-tiered process the projected enrolments (for SA1s) are calculated using the 30 September 2016 relationship between each SA1's enrolments and its ERP (see Appendix III).
The lack of demographic data collected regularly at SA1 level makes it necessary to use the conversion method as outlined above. While the process is quite complex, it should be reiterated that the basic concept of splitting SA2s to SA1 level cannot be expected to give projections as reliable as those for SA2s. However, as the goal is to support the redrawing of State Electoral Division boundaries which are aggregates of large numbers of SA1s there is a high likelihood that any random errors or inconsistencies will be statistically offset in the aggregation process.
Previous redistributions have used Census Collection Districts (CCDs) as the base unit, however in 2011 CCDs were superseded by the new SA1 unit.
SA1 and SA2 boundaries are from the Australian Statistical Geography Standard (ASGS) Volume 1 – Main Structure and Greater Capital City Statistical Areas, July 2011 (ABS Cat. 1270.0.55.001) corresponding to those used for the 2011 Census.
Further detail, including algorithms is available as PDF documents here.
It is important to recognise that the projection results given in this report 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.
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 Tasmanian 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.
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