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APPENDIX 3 USING HISTORICAL ESTIMATES OF THE ABORIGINAL AND TORRES STRAIT ISLANDER POPULATION FOR COMPARATIVE ANALYSIS AND REPORTING PURPOSES
Other collections, such as surveys and administrative datasets also rely on a person's Indigenous status to be accurately reported and recorded, and therefore may likewise be susceptible to differing rates of identification, which may also vary over time (endnote 3). The changing rates of identification of Aboriginal and Torres Strait Islander people in the Census and other datasets create challenges when comparing data about this population over time.
COMPARISON OF HISTORICAL SERIES
After each Census, the ABS creates a time series for the population based on that Census count, by projecting and backcasting around this estimate. The backcast series shows what the Aboriginal and Torres Strait Islander population (based on the latest Census) would have been in previous periods, based purely on demographic change. Similarly, the projected series shows how the currently identified Aboriginal and Torres Strait Islander population may change in future years, if certain demographic assumptions were to hold. Each Census-based series is independent from the others.
Graph A3.2 shows the five different population series that cover the period 1991-2011, as well as how each year's estimated population would change if a simple, straight line interpolation was applied between successive Census-based estimates.
Often, population estimates for Aboriginal and Torres Strait Islander people are used as the denominator for rates, to compare data for the Aboriginal and Torres Strait Islanders population with the non-Indigenous population, or to create time-series indicators about how various aspects of the well-being of Aboriginal and Torres Strait Islander people are changing over time. The use of different series of population estimates shown in graph A3.2 will clearly impact on such indicators.
Following are some examples of the impact that the choice of Aboriginal and Torres Strait Islander population estimates can have on a rate. The examples use six different series to demonstrate this:
Example 1: Child Mortality Rates
The use of different Census-based Aboriginal and Torres Strait Islander population estimates, while showing a similar pattern of change over time, has a considerable impact on the level of historical child mortality rates, as shown in graph A3.3. When using a combination of series, as shown in graph A3.4, the downward trend in the rate is more obvious.
Example 2: School Enrolments
In the case of school enrolments data, the use of previous Census-based series generates impossible results, with rates exceeding 100%. In contrast, when using a combination of Census-based population series (graph A3.6), the enrolment rates remain plausible throughout, although there is no longer a clear trend in the time-series. Further analysis would be needed to determine the most appropriate choice of Aboriginal and Torres Strait Islander population estimates. In particular, the choice may depend on an understanding of how identification rates in the enrolment data (numerator) had changed over time.
Example 3: Standardised Death Rates
In the case of age-standardised death rates, the use of different Aboriginal and Torres Strait Islander population estimates changes the direction of the trend, as seen in the 2001-based series. This change between the 2001-based and 2006-based data was the result of an improvement to ERP - specifically, using date of birth information from the 2006 Census to derive better estimates of the population at the older age groups in 2006 compared with 2001. For this reason, it is always recommended not to use 2001-based estimates when reporting on older age groups.
In each of the examples above, of the three methods of transition between population estimate series shown, the rate time-series is smoothest when using a linear interpolation between Census year estimates. Changing from one Census-based series to the next results in a sharp drop in the rate at the changeover point. This can be seen around 2003 and around 2008, with the later transition being far more significant. 2013 would be the equivalent 'decision point' for the current intercensal period. The extent of the 2013 transition will not be known until after the 2016 Census-based ERP series becomes available.
WHICH SERIES TO USE
The question of which historical population series to use depends on the purposes of the analysis. The following provides guidelines to help in making these decisions.
The 2011-based series is the most accurate and up-to-date series currently produced by the ABS, including in terms of the methodology used in the Census collection and in the estimation/projection processes. Wherever possible, the 2011-based series should be used. This includes any of the following scenarios:
Using a range of different Census-based population estimate series could be considered when:
In some cases, an alternative data source may exist which is more comparable/consistent with the numerator data, and could be used rather than a population estimate series. For example,
If in any doubt, the ABS recommends that data users conduct their own analysis, for example similar to that illustrated in the examples above. This will highlight the impact of non-demographic changes on their particular area of interest.
Impact of Future Changes
A new series for Aboriginal and Torres Strait Islander population estimates will be produced following the 2016 Census. Depending on how the 2016 Census-based estimate compares with the 2011 Census-based estimate, current indicator trends may change as a result of rebasing any population estimates to the new Census.
Aboriginal and Torres Strait Islander population projections are produced to illustrate potential future demographic changes to the currently identified population. The 2011-based projections therefore do not include any assumption variable on propensity to identify. Anyone considering the future population should consider their requirements for a projection based on a change in propensity to identify, and note that the ABS projections cannot be used for this purpose.
(1) The ACLD links a 5% sample of Census records from the 2006 to 2011 Census. The ACLD can be accessed from https://www.abs.gov.au/websitedbs/censushome.nsf/home/acld?opendocument&navpos=267
(2) For more information on the increase between the 2006 and 2011 Census counts, see Census of Population and Housing: Understanding the Increase in Aboriginal and Torres Strait Islander Counts, 2006-2011 (cat. no. 2077.0). For information on the increase between 1991 and 1996, see Occasional Paper: Population Issues, Indigenous Australians, 1996 (cat. no. 4708.0)
(3) For further discussion of how and why identification differs between contexts, see Information Paper: Perspectives on Aboriginal and Torres Strait Islander Identification in Selected Data Collection Contexts, 2012 (cat. no. 4726.0)
(4) For an example of data linkage, see Information paper: Death registrations to Census linkage project - Key Findings for Aboriginal and Torres Strait Islander peoples, 2001-2012 (cat. no. 3302.0.55.005)
(5) For more information on the difference between Census counts and Census year population estimates, see Estimates of Aboriginal and Torres Strait Islander Australians, June 2011, Technical Note 1: Estimated Resident Aboriginal and Torres Strait Islander Population - Method of Calculation (cat. no. 3238.0.55.001).
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