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3302.0.55.002 - Discussion Paper: Assessment of Methods for Developing Life Tables for Aboriginal and Torres Strait Islander Australians, 2006  
Latest ISSUE Released at 11:30 AM (CANBERRA TIME) 17/11/2008   
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INTRODUCTION

In this discussion paper, the word "Indigenous" refers to Aboriginal and Torres Strait Islander peoples of Australia. We thank Australian Aboriginal and Torres Strait Islander peoples for their cooperation and assistance in the collection of data, without which, this analysis would not have been possible.

Life expectancy estimates for Aboriginal and Torres Strait Islander Australians are an important aspect of assessing Indigenous disadvantage. The compilation of accurate life tables to derive life expectancy estimates for Aboriginal and Torres Strait Islander (Indigenous) Australians presents particular difficulties. Specifically, the standard approach to compiling life tables and resulting life expectancies at birth requires complete and accurate data on deaths that occur in a period, and an estimate of the population exposed to those deaths at the mid-point of the period. These data are required by age and sex. In the case of Indigenous mortality estimation, this situation is far from being perfect. Both Indigenous population estimates and death registrations have limitations.

Despite these limitations, the importance of life expectancy estimates and population projections for the purposes of planning, policy and program formulation, evaluation, research, analysis, and resource distribution purposes are well recognised by the Australian Bureau of Statistics (ABS).


PURPOSE OF THIS DISCUSSION PAPER

The ABS has previously compiled experimental life tables for Indigenous Australians following the 1996 and 2001 Censuses of Population and Housing. Those estimates were compiled using different indirect demographic methods and were subject to a range of caveats. Because of these caveats, the estimates have been labelled 'experimental'. In spite of those caveats, the resulting estimates have been used extensively.

In considering the compilation of life tables for Indigenous Australians following the 2006 Census, the ABS has been investigating a range of different methodologies. These investigations have once again highlighted the sensitivity of these estimates to the underlying assumptions, input data and methods explored.

The objective of this Discussion Paper is to highlight the issues associated with compiling life tables and life expectancy estimates for Indigenous Australians. The paper discusses the different methods available, the data limitations and the resulting outcomes for the different methods. An ABS preferred approach of using a direct demographic method, by adjusting the death registration data by the undercoverage factor obtained from the Census Data Enhancement (CDE) Indigenous Mortality Quality Study, to derive Indigenous life tables is presented.

The purpose of the Discussion Paper is to give users and stakeholders the opportunity to provide feedback and to ensure all issues associated with compiling life tables and life expectancy estimates for Indigenous Australians are adequately considered before finalising the Indigenous life tables in early 2009. A consequence of this approach will be that experimental Indigenous life tables and life expectancy estimates for 2001-2006 will not be included in Deaths, Australia, 2007 (cat. no. 3302.0) scheduled to be released on 25 November 2008. The ABS will advise on the specific timing of the release of experimental Indigenous life tables and life expectancy estimates in due course.

It should be noted that the Indigenous life expectancy estimates presented in this Discussion Paper are preliminary in nature. The ABS will continue to review the estimates and the methodology used prior to publication of the final estimates.


METHODS ASSESSED

The ABS has assessed four methodologies for compiling Indigenous life tables and life expectancies. These can be broadly grouped into indirect demographic methods and direct demographic methods.

The indirect methods (Bhat with and without unexplained growth, and Hill), which are discussed in detail in Chapters 3 and 5, require Indigenous population estimates by age and sex, at two time points and the recorded Indigenous deaths in the intervening period. The methods effectively derive an estimate of undercoverage in the recorded deaths which are adjusted accordingly, with resulting life tables and life expectancies. The outcomes for life expectancy under each of these models is summarised below:

1.1 Indigenous Life Expectancy at Birth, Australia, Indirect Methods - 2001-2006

Males
Females
Method
years
years

Bhat without unexplained growth
62.5
71.0
Bhat with unexplained growth
61.7
69.3
Hill
63.2
71.7


The investigations undertaken by the ABS have identified two main concerns with the use of indirect methods. First, for both the Bhat and Hill methods the resulting estimates of coverage of Indigenous deaths for 2001-2006 are markedly lower (20-25 percentage points) than the coverage rates for the 1996-2001 period, implying that coverage of Indigenous deaths in death registration data has worsened considerably over the last five years. Such a result would seem implausible when compared to the observed data. Second, analyses undertaken by ABS and others (Barnes et al., 2008) have highlighted that the life expectancy estimates are very sensitive to the quality of the population estimates at the respective end points. Errors in the age distribution and level of either population estimate can result in very different life expectancy outcomes.

Based on the analysis of the 2006 data, the ABS is of the view that the indirect demographic methods are not suitable to the Australian situation and no longer recommends their use for the purpose of compiling Indigenous life expectancy estimates.

The direct demographic method is the method used for deriving life expectancy for the Australian population without any adjustment to deaths data. It takes the population at a point in time and the registered deaths around that period (e.g. the average of three years to reduce the effect of year-to-year variation) to compile life tables and life expectancy estimates. The method assumes good quality population estimates and assumes complete coverage of deaths.

The direct method can also be applied with a prior adjustment to the deaths or population data based on assumptions regarding coverage. Previously the ABS only had the indirect methods for deriving coverage estimates in respect of deaths. However, in 2006 a more direct measure of coverage was possible using results from the Census Data Enhancement (CDE) Indigenous Mortality Quality Study. This study linked death records for an 11 month period to the 2006 Census records, which enabled comparison of the reporting of Indigenous status in the two records. More detail on the CDE Indigenous Mortality Quality Study is provided in Chapters 4 and 5 of the Discussion Paper. In summary, the study indicated that the coverage of Indigenous deaths in death registration data was higher than previously estimated (based on the indirect methods).

The outcomes for Indigenous life expectancy for 2005-2007 using direct methods are presented below. The first series (Unadjusted) is based on no adjustment to deaths data, i.e., it is assumed that death registration data are complete and is provided primarily for completeness of analysis. The second series (CDE adjusted) is derived by adjusting the death registration data by the undercoverage observed in the CDE Indigenous Mortality Quality Study.

1.2 Indigenous Life Expectancy at Birth, Australia, Direct Methods - 2005-2007

Males
Females
Method
years
years

Unadjusted
68.4
73.8
CDE adjusted
66.9
72.6


While the results using the CDE Indigenous Mortality Quality Study do result in some outcomes that are difficult to reconcile (e.g. the results for both WA and NT indicate that death registrations represent an overcoverage of Indigenous deaths relative to Indigenous status in the Census), they do represent the only direct measure of undercoverage of Indigenous deaths available.


ABS PREFERRED APPROACH

The ABS is of the view that a direct method using the CDE Indigenous Mortality Quality Study presents the best option for deriving measures of Indigenous life expectancy for 2006. The approach based on the CDE Indigenous Mortality Quality Study does have limitations. However, it is data based and does not require assumptions. Also, confidence intervals around any error in the estimates can be established. It also produces results that are considered to be more plausible, particularly in regard to the coverage of death registrations.

As indicated above, the indirect methods require extensive assumptions and produce significantly implausible results in terms of outcomes in relation to the apparent undercoverage measures over time. Furthermore, the analysis undertaken has highlighted the sensitivity of the estimates to the accuracy of the population estimates.

The preliminary results for 2006 by state/territory (for those for which estimates can be reasonably compiled) using the preferred approach are presented in the table below.

1.3 Indigenous Life Expectancy at Birth, CDE Indigenous Mortality Quality Study - 2005-2007

Males
Females
State/territory
years
years

NSW
69.6
74.8
Qld
68.1
73.5
WA
64.9
70.2
NT
61.5
69.2
Aust.(a)
66.9
72.6

(a) Includes all states/territories.



COMPARISON TO NON-INDIGENOUS LIFE EXPECTANCIES

Table 1.4 presents the difference between Indigenous life expectancy by state/territory (for which data can be compiled) and the Australia level non-Indigenous life expectancy using the CDE Indigenous Mortality Quality Study to adjust Indigenous death registration data.

1.4 Indicative difference in life expectancy between Indigenous and non-Indigenous Australians - 2005-2007

Indigenous life expectancy(a)
Indicative difference(b)
State/territory
years
years

Males

NSW
69.6
9.1
Qld
68.1
10.6
WA
64.9
13.8
NT
61.5
17.2
Aust.(c)
66.9
11.8

Females

NSW
74.8
7.8
Qld
73.5
9.1
WA
70.2
12.4
NT
69.2
13.4
Aust.(c)
72.6
10.0

(a) Using CDE adjusted approach.
(b) Difference between Australia level non-Indigenous life expectancies (78.7 years for males and 82.6 years for females) and state/territory level Indigenous life expectancies.
(c) Includes all states/territories.


The results indicate a difference at the Australia level of 11.8 years for males and 10.0 years for females. The estimates vary considerably at the state/territory level with NT (17.2 years for males; 13.4 years for females) being the highest, while NSW (9.1 years for males; 7.8 years for females) was the lowest of the states for which estimates could realistically be compiled. At the same time the standard errors and other limitations (see Chapter 5) associated with these estimates need to be clearly kept in mind.


COMPARISON WITH PREVIOUS PERIODS

The above Indigenous life expectancy estimates are markedly higher than the experimental estimates published for 1996-2001, which were based on an indirect method (Bhat with unexplained growth). In terms of trying to assess changes in life expectancy over time, it needs to be recognised that the above concerns identified in relation to indirect methods are equally applicable to the ABS' previously published Indigenous life expectancy estimates for the period 1996-2001. While the ABS took considerable care to explain the underlying assumptions, and that the estimates were of an experimental nature, it would seem clear now that the derived coverage rates and resulting life expectancy estimates for some states/territories are implausible. The ABS would strongly advise against any comparison of the published Indigenous life expectancies based on 1996-2001 with the estimates presented in this Discussion Paper, and in no way should those differences be interpreted as measuring changes in life expectancy over time.

While the CDE Indigenous Mortality Quality Study does not provide a basis for deriving life tables and life expectancy estimates for previous periods, a range of possible outcomes for previous periods using different assumptions regarding undercoverage of Indigenous deaths in 2000-2002 are presented in Chapter 5 of this Discussion Paper. The results would suggest that the use of indirect methods for 1996-2001 may have resulted in an understatement of Indigenous life expectancy at birth for some states/territories and at the Australian level. For example, if death coverage was 20 percentage points less for 2000-2002 relative to the CDE estimates for 2005-2007, then the life expectancy estimate using a direct adjustment method would be 1.8 years higher for males and 2.4 years higher for females than the previously published ABS estimates (59.4 years for males and 64.8 years for females). Unfortunately, it is not possible to quantify the change in death coverage over the time frame.


FUTURE DIRECTIONS

The analysis undertaken for 2006 has highlighted the complexities in compiling Indigenous life tables and life expectancy estimates. It is clear that the estimates are subject to a range of potential errors in terms of data inputs, for example, the population estimates are subject to the sampling error associated with the Post Enumeration Survey, and the estimates of undercoverage in death registration data are subject to under or over estimation. In that context it needs to be recognised that life expectancy estimates are just that, estimates based on the available information.

The analysis for 2006 has highlighted the limitations of indirect demographic methods in the compilation of Indigenous life tables and life expectancy estimates. Furthermore, it has emphasised the importance of having direct measures of the extent of undercoverage of death registrations data to provide a basis for adjustment in the compilation of these estimates. The ABS would see such measures as being integral to the compilation of future life tables and life expectancy estimates. The ABS would view a repeat of the CDE Indigenous Mortality Quality Study as critical to the compilation of Indigenous life tables and life expectancy estimates following the 2011 Census of Population and Housing.

Needless to say the difficulties associated with the compilation of life tables and life expectancy estimates are exacerbated when trying to monitor changes over time.


OUTLINE OF SUBSEQUENT CHAPTERS

The remaining chapters of the Discussion Paper cover the following aspects.

Chapter 2 describes the requirements for the compilation of accurate life tables and details the limitations of the population and death estimates for Indigenous Australians, and the need to adopt alternative methods to adjust Indigenous death registrations to account for undercoverage.

Chapter 3 presents discussion of the indirect methods considered in the compilation of life tables and life expectancy estimates. In particular, it discusses the derived coverage rates arising from the methods and the suitability of their use in the Australian context.

Chapter 4 describes a direct demographic method using information from an ABS data linkage project as the basis for adjusting for undercoverage in Indigenous death registrations.

Finally Chapter 5 discusses the outcomes in terms of life expectancy estimates that result from the use of the different methods and presents the ABS preferred method.


ISSUES FOR CONSIDERATION

ABS would welcome comments on any aspects of this Discussion Paper but in particular would appreciate comments as to:
  • whether the proposed direct method is supported given the available options,
  • any suggestions to help improve the suggested method,
  • any other evidence available as to the extent of undercoverage of Indigenous deaths in death registration data,
  • any other aspects of the analysis presented.

Comments should be forwarded by 15 December 2008 to:

Director,
Demography Section
Australian Bureau of Statistics, Locked Bag 10
BELCONNEN ACT 2616
Phone: (02) 6252 6411

or

E-mail: demography@abs.gov.au

In addition, the ABS will be contacting key stakeholders to discuss issues identified in this paper.


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