Life expectancy refers to the average number of years a person of a given age and sex can expect to live, if current age-sex-specific death rates continue to apply throughout his or her lifetime. A 'life table' is created from age-specific death rates that are used to calculate values which measure mortality, survivorship and life expectancy. To construct a life table, data on total population, births and deaths are needed, and the accuracy of the life table depends upon the completeness of these data. Because of uncertainty about the estimates of these components for Aboriginal and Torres Strait Islander peoples, indirect experimental methods are used to calculate life expectancies for the Indigenous population. These experimental life expectancies should only be used as an indicative summary measure of the level of mortality of the Indigenous population.
The estimates of life expectancy presented here are drawn from the Australian life tables, 1998-2000, and the Experimental Indigenous Abridged Life Tables, 1996-2001 (ABS 2006b) which use the Bhat method (ABS 2004b) to estimate life expectancy (see box 9.3, below) Life expectancy estimates are not available for the non-Indigenous population so estimates for the total Australian population have been used for comparison with estimates for the Indigenous population. The life expectancy estimates presented here are the same as those presented in the 2005 edition of this report. This is because at present, Indigenous life expectancy estimates have not been calculated for a later period than 1996-2001. Life expectancy estimates for the total Australian population are available for the period 2004-2006, however, for comparative purposes, 1998-2000 data for the Australian population have been used in this chapter.
In the period 1996-2001, the life expectancy at birth for Indigenous Australians was estimated to be 59 years for males and 65 years for females, compared with 77 years for all males and 82 years for all females for the period 1998-2000; a difference of approximately 17 years for both males and females.
|9.3 METHODS USED TO CALCULATE LIFE EXPECTANCY|
A number of different indirect methods have been used to estimate life expectancy of Indigenous Australians. All of these methods rely on different assumptions and subjective expert opinions (ABS 2004b) and there is no direct way of verifying the accuracy of the estimates derived from these methods. More work needs to be done on such estimates as more robust methods become available and data quality improves. Below is a description of the methods used.
Preston and Hill (1980)
The ABS first used a method proposed by Preston and Hill (1980) to estimate the completeness of recording of deaths of Indigenous Australians in the national mortality database relative to the number of Indigenous Australians recorded in the five-yearly Census of Population and Housing. The Preston-Hill method yields correction factors which adjust the counts of deaths recorded during the intercensal period so that the census population estimates at each end of the period are consistent with corrected intercensal death registration. In this method, net internal migration (by age, sex and state/territory) and net overseas migration (by age and sex) are assumed to be nil, and no adjustment is made for change in the extent to which people were identified as Indigenous in the census. In a review of the performance of the method, the ABS determined that the method was not appropriate for application to the Indigenous population because the method only allows for stable populations (ABS 2004a) which is not the case for the Indigenous population. The 1980 Preston-Hill method has been used extensively worldwide and was used by the ABS to estimate Indigenous mortality for the period 1991-1996. The ABS application of the Preston-Hill method produced estimates of life expectancies at birth of 57 years for Indigenous males and 62 years for Indigenous females for 1991-1996. Indigenous life expectancy estimates using the Preston-Hill method have also been calculated for the periods 1997-99 and 1999-2001, resulting in an estimated life expectancy for Indigenous males of 56 years and for Indigenous females of 62 years for both periods. These estimates are around 20 years lower than the life expectancy estimates derived for all Australian males and females.
Following the 2001 census, the ABS shifted to a method proposed by Bhat (2002) that offers improvement over other indirect methods used earlier by the ABS to estimate life expectancy from incomplete data. The Bhat method has advantages over other methods in that it allows for an adjustment for 'migration' which is used to allow for the 'unexplained growth' of the Indigenous population which is attributed to a changing propensity to identify as Indigenous between censuses. However it should be noted that this method was primarily developed for population estimation and requires information about the rate of natural increase of the population and remains experimental. The application of the Bhat method, assuming 2.0% growth per annum during the 1996-2001 period, produced life expectancy estimates of 59 for Indigenous males and 65 years for Indigenous females, about 17 years lower than those estimated for all Australian males and females.
General Growth Balance Method (GGB) (2007)
The GGB method is similar to the Bhat method however it treats change in identification as change in census coverage without an additional adjustment for 'identification migration', i.e. assuming a population growth rate. The GGB method was used by the University of Queensland in the 2007 report 'The burden of disease and injury in Indigenous Australians, 2003'. This report estimated life expectancy at birth for Indigenous Australians around 13 years lower than that of the total Australian population (64 years for Indigenous males and 69 years for Indigenous females compared with 77 years for all males and 82 years for all females.)