APPENDIX 3 USING HISTORICAL ESTIMATES OF THE ABORIGINAL AND TORRES STRAIT ISLANDER POPULATION FOR COMPARATIVE ANALYSIS AND REPORTING PURPOSES


INTRODUCTION

This appendix outlines the issues to consider in choosing an appropriate series of Aboriginal and Torres Strait Islander population estimates, for the purpose of analysing or reporting on this population over time.


WHY THE ABS RECASTS ABORIGINAL AND TORRES STRAIT ISLANDER POPULATION ESTIMATES AFTER EACH CENSUS

The Census of Population and Housing (Census) provides the basis of Australia's official population estimates. For the total Australian population, quarterly estimates can be produced between Censuses by applying components of population growth (births, deaths and migration) to the latest Census-based estimate. Generally speaking, the accuracy of the Census and component data means that only minor adjustments to the population figures occur after each Census.

However, Aboriginal and Torres Strait Islander Census counts and the quality of data on births, deaths and migration of Aboriginal and Torres Strait Islander people do not support the use of the standard approach to population estimation. Instead, the ABS uses assumptions about future fertility, paternity, life expectancy and migration to project figures out from each Census. At the same time, it is also necessary to backcast the Aboriginal and Torres Strait Islander population, because each Census count is sufficiently different from the last.

The uncertainty in the Census and component data occurs because the Aboriginal and Torres Strait Islander population is very small compared with the total Australian population (around 3%), and because the Aboriginal and Torres Strait Islander population requires people not only to be counted in the Census and component datasets, but also to be identified as being of Aboriginal or Torres Strait Islander origin.

The differences between the Aboriginal and Torres Strait Islander population estimates derived from each Census thus tend to be larger than demographic changes can account for. Each Census-based estimate reflects both the desire of people at that time to be identified as being of Aboriginal or Torres Strait Islander origin, and the ability at that time for the information to be effectively collected in the Census.

The Australian Census Longitudinal Dataset (ACLD) also suggests that a certain proportion of the population will be counted as Aboriginal or Torres Strait Islander in one Census, but as non-Indigenous in the next, and vice versa (endnote 1).

Graph A3.1, below, shows the percentage increase in estimated resident population (ERP) between Censuses for both the Aboriginal and Torres Strait Islander population and the total Australian population, for the last five intercensal periods. The change in the total population between Censuses remains lower than 10%, and is largely consistent with birth, death and migration rates observed between Censuses. This is not the case for the Aboriginal and Torres Strait Islander population, for which implausible demographic trends would need to have occurred to account for the intercensal difference.

Of the increase in the Aboriginal and Torres Strait Islander population observed between the 2006 and 2011 Censuses, the ABS estimates that 43% was attributable to demographic change. An estimated 36% of the increase was due to methodological improvements, with propensity to identify a major contributor to the remaining 21% unexplained increase (endnote 2).

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:

• the 2011 Census-based series
• the 2006 Census-based series
• the 2001 Census-based series
• a series which uses each Census-based series for the two years before and after that Census (option 1)
• a series which transitions between Census-based series in a way that minimises the 'jump' in the transition year (option 2)
• linear interpolation between Census estimates.

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:

• for demographic analysis of the Aboriginal and Torres Strait Islander population
• for forward-looking analysis (i.e. from 2011 onwards), for any purpose
• as the denominator of rates being compared over time, if:
  • the time series is short, generally no further back than the penultimate (2006) Census
  • the numerator data are known to have been fairly consistently high over time
  • analysis is restricted to remote geographies, where the impact of increased identification is much lower than in non-remote areas (8% compared with 24% increase in the 2011 Census)
  • when there is a known quality issue in the data from earlier Censuses (such as the older age groups in the 2001 Census, mentioned above).

Using a range of different Census-based population estimate series could be considered when:

• transitioning from one series to the next will maximise consistency of identification levels and coverage between the numerator and the denominator
• historical rates are thought to be artificially deflated when using the 2011-based population estimates (usually due to lower identification rates in the numerator series in earlier years)
• there is a known relationship between identification in the numerator data and the Census data. Data linkage may be used to inform on this relationship (endnote 4). This relationship may be applied to the 2011-based series as a factor, or be used to inform the choice of a transitional series
• data confrontation using supplementary data sources indicates that the 2011-based population estimates do not produce a plausible result.

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,

• survey data should be compared to the survey population. That is, re-weighting past surveys to the latest population estimate series is not generally recommended
• analysis of Census data should be based on Census counts rather than population estimates (endnote 5)
• numerator data could be compared to a denominator that is internally consistent, rather than to population estimates - for example retention rates (15 year-old students in 2013 as a percentage of 12 year-old students in 2010) produces a more coherent time-series than enrolment rates (15 year-old students as a percentage of 15 year-old population).

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.


ENDNOTES

(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).