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Abstract:
Estimates and projections of the Australian Aboriginal and Torres Strait Islander (Indigenous) population are required for a wide range of planning, policy development, evaluation, research and analysis purposes. A key component of the estimation/projection process is the construction of experimental Indigenous life tables. This paper therefore focuses on the life table construction process, and its associated assumptions. The ABS assesses that these life tables allow the best experimental Indigenous population estimates and projections using currently available data. The Indigenous life expectancy estimates presented in this paper are experimental because of the nature of the base population estimates, which are affected by both intercensal volatility in the counts of the Indigenous population and deficiencies in Indigenous identification in birth and death registration data. Consequently, there is uncertainty about the accuracy of death rates which can be derived from these inputs and used in life table construction. While the life expectancy estimates are the best that can be compiled with currently available data, and are assessed to be suitable for experimental population estimates and projections, over-precise analysis of the life expectancy estimates as measures of Indigenous health outcomes should be avoided. In particular, the differences between the life expectancy estimates in this paper and those previously published by the ABS, which were based on analysis of data for the intercensal period 1991 to 1996, represent improvements in methods and data quality and do not necessarily represent any change over time in the life expectancy of the Indigenous population. The standard approach to calculating death rates used in life tables is to divide the number of deaths in a given population by the population 'exposed to risk' during that period. Precisely estimating the population at risk between census years and in the projection period requires accurate information on births. Although it is considered likely that most Indigenous births and deaths are registered, a proportion of Indigenous births and deaths are not identified as ‘Indigenous’. There have also been intercensal increases in the Indigenous population that cannot be explained solely by demographic factors, i.e. by biological natural increase (the excess of births over deaths), or by migration to/from Australia. Without complete and accurate data on Indigenous births and deaths, and accurate data on the size and structure of the Indigenous population, the standard method for calculating death rates cannot be used for the Indigenous population. This paper describes a new demographic method for determining consistency factors to make registered intercensal deaths data consistent with two-end census date population estimates. This method was developed by Bhat (2002) and offers improvement over other indirect methods available for estimating mortality from incomplete data. The main advantage of the Bhat method over other methods is that it explicitly allows for an adjustment for 'migration' to be taken into account. The migration component in the Bhat method is being used in this paper to allow for the non-biological, or unexplained, change in the measured Indigenous population. Using the Bhat method, this paper calculates the consistency of Indigenous death registrations for the 1991-1996 and 1996-2001 intercensal periods relative to population estimates at the beginning and end of each five year period. A consistency factor, assumed to be constant at each age group, is applied to the observed age-specific death rates, which are then used for the calculation of the Indigenous life tables. While life table construction based on the Bhat method provides an essential component for preparing experimental population estimates and projections for Aboriginal and Torres Strait Islander Australians, the use of the life tables for other purposes should be undertaken with caution and an understanding of the assumptions used in their construction. This paper assesses the sensitivities of the method to assumptions about the level and age distribution of unexplained growth in the Indigenous population. The sensitivity analysis is restricted to the 1996-2001 period only. Results obtained from the sensitivity analysis show that life expectancy estimates derived under various assumptions vary widely, suggesting that the life expectancy estimates in the life tables will not be robust for some forms of analysis. While the Bhat method overcomes one of the deficiencies inherent in previous methods used to construct experimental Indigenous life tables published by the ABS, it cannot overcome the underlying issues of quality in the input data.
