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6.1. ABS produces a range of population projections, both published and unpublished. ABS commenced its involvement in this area in the late 1940s and began publication in the late 1970s. Published projections relate to the total population down to capital city/balance of State level and for the Indigenous population down to the State level. In late 1999 projections by household composition will also be published. Unpublished projections using customised assumptions are also produced for populations that include the total population, the Indigenous population and the electoral roll population. Such unpublished projections are not official ABS statistics and should generally be referred to as projections produced the ABS according to assumptions agreed to by (the client).
6.6. Survivorship rates (1 - death rates) are applied to the population thereby 'ageing' it one year, for ages 0 to max - 1. Overseas migrants are assumed to arrive on average half-way through the projection year, so (a) they only have half a year's mortality rates applied, and (b) are half an age older than Px, therefore requiring new ages to be formed by combining two halves of consecutive ages.
6.7. A similar process is performed for the highest age group:
6.8. Births are then calculated by applying calendar year fertility rates to the mid year population. As births will occur on average half-way through a projection year (ie. t+½), the required births are the average of when fertility rates are applied to year t and year t+1 populations.
6.9. Population aged zero is then calculated by 'surviving' the births (from t+½ to t+1) and adding 'survived' overseas migration.
6.10. Where interstate migration is required, it is calculated by applying departure rates to a state's population (those at risk of departing) and arrival rates to the rest of Australia's population (those at risk of moving to that state). These rates are derived from 1996 Census data and are held constant for the duration of the projection.
6.11. The resulting total arrivals and departures are then scaled to the pre-determined net interstate migration assumption. Finally, the arrivals and departures by age and sex are scaled to the new arrival and departure totals (while ensuring they net across all States to give zero for Australia), then combined to give net interstate migration.
6.12. Then add the interstate migration:
6.13. A final proration is then undertaken to ensure lower geographic levels sum to the projected age/sex population of any higher geographic level. Year t+1 then becomes the base for projecting the next year and the cycle is repeated until the final projection year is reached.
Experimental Projections of the Indigenous Population
6.14. ABS projections of the Aboriginal and Torres Strait Islander (Indigenous) population are referred to as experimental because of the experimental nature of the base population and the deficiencies in the quality of Indigenous births, deaths and migration data involved in deriving the population projection assumptions. The inclusion of an assumption for change in propensity to identify as Indigenous on a census form also adds to the experimental nature of the projections. The most recent edition is Experimental Projections of the Aboriginal and Torres Strait Islander Population, 1996 to 2006 (3231.0).
6.18. The projected low-growth scenario assumed no future change from the 1996 propensity to identify as Indigenous. The high-growth scenario assumed that the average annual change in propensity to identify between 1991 and 1996 Censuses will continue. This adjustment to the population was made at the end of each year's projection cycle (ie. after (8) above) thus:
6.19. Unlike in general population projections where the number of births can be expressed entirely as a function of the number of women in childbearing ages, Indigenous births may also come from Indigenous men and non-Indigenous women. These birth rates are referred to as 'paternity rates', while births to Indigenous women are, as usual, referred to as fertility rates. Assumed paternity rates are applied in the projection at the same stage as fertility rates (ie. at formula (3) above) using an equivalent formula: