Method: Calculating life expectancy

Method used in HealthStats NSW

For HealthStats NSW, estimates and confidence intervals for life expectancy were calculated using abridged current life tables based on five-year age groups, except for the first five years of life, which were split into two age groups: 0-under 1 year and 1-4 years. The methods used are described in detail by Chiang (Chiang 1968 and Chiang 1984) but include a modified variance calculation which is able to accommodate for zero deaths within age bands that precede the final age interval. This modified method is referred to as Chiang II with a further adjustment in the variance calculation in the final age interval using a method from Silcocks (Slicocks et al, 2001). For full details see Stephens et al (2013).

Australia's life expectancy is ranked among the highest in the world. Methods used to calculate life expectancy differ between countries, resulting in slight differences in the final figures reported by countries. These differences are not considerable and the group of countries that leads has been stable for a number of years, however their ranking may change somewhat from year to year.

Differences between methods depend on:

Life tables

A life table is a statistical method used to represent mortality of a population. In its simplest form, a life table is generated from age-specific death rates and the resulting values are used to measure mortality, survivorship and life expectancy.

One of the differences between methods to calculate life expectancy is the use of complete or abridged life tables, resulting in different age intervals used in the calculations. For example, the ABS publishes complete life tables for the Australian population (that is, the tables contain data by single years of age) and abridged tables (for five-year age groups) for the Aboriginal and Torres Strait Islander population.

The ABS implements graduation (Lidstone transformation) and Brass’ Logit system for the estimation of life expectancy for State and Territory and for Statistical Area Level 4 and Statistical Division levels of geography respectively (3302.0 - Deaths, Australia, Explanatory Notes, ABS).

Smoothing techniques

Another difference between methods stems from using one year or several combined years of deaths data as a smoothing technique designed to reduce the effect of year-to-year data variations.

The ABS uses three combined years of death data. The one year technique is used in HealthStats NSW.

Other differences

Other differences involve the exclusion, or otherwise, of residents who are physically not present at the time of count and actuarially graduating the tables. Both these adjustments are applied by the ABS (ABS 3302.0 2009).

The World Health Organization (WHO) calculates and reports estimates for all countries based on the information about population, births and deaths supplied by the countries. In this instance, although the method used is the same for all countries, the results are limited by the accuracy and timelines of information supplied by the countries.

Change in methods to calculate life expectancy in Australia’s Aboriginal population

Constructing a life table requires data on deaths and total population. Births data are also needed for adjusting the size of the total population. Because of uncertainty about the estimates of these components for Aboriginal peoples, indirect experimental methods were previously used by the ABS to calculate life expectancies for the Aboriginal population in Australia. The ABS reviewed available methods in 2008 with the view of using the one most appropriate for the circumstances of increasing self-identification. Three different methods that could be used to estimate life expectancy and which are discussed in ABS publications (ABS 3302.0.55.002 2008; AIHW 2010; ABS Cat. no. 3302.0.55.003 2013) gave different results, ranging from 20 years lower than the life expectancy estimates derived for all Australian males and females to 17 years and 13 years lower. The ABS has concluded that the previously used (indirect methods) are inappropriate for Australia and direct methods are now used with Aboriginal deaths data augmented through linkage to the ABS census data (ABS Cat. no. 3302.0.55.003 2013).

References

Chiang C L. The life table and its construction. From: Introduction to stochastic processes in Biostatistics. New York: John Wiley & Sons 1968, Chapter 9, 189–214.

Chiang C, Long. The life table and its applications. Malabar: Krieger Publishing Company, 1984.

Silcocks PB, Jenner DA, Reza R. Life expectancy as a summary of mortality in a population: statistical considerations and suitability for use by health authorities. J Epidemiol Community Health 2001; 55(1): 38–43.

Australian Bureau of Statistics. Deaths, Australia, Latest Year. Canberra: ABS,. Available at https://www.abs.gov.au/statistics/people/population/deaths-australia/latest-release

Australian Institute of Health and Welfare. Life expectancy. AIHW. Accessed 17 May 2022. Available at https://www.aihw.gov.au/reports/life-expectancy-death/deaths-in-australia/contents/life-expectancy

Australian Bureau of Statistics Life tables for Aboriginal and Torres Strait Islander Australians 2015-2017 Canberra: ABS Accessed 17 May 2022. Available at https://www.abs.gov.au/statistics/people/aboriginal-and-torres-strait-islander-peoples/life-tables-aboriginal-and-torres-strait-islander-australians/latest-release

Australian Bureau of Statistics. Experimental life tables for Aboriginal and Torres Strait Islander Australians 2010-12. 3302.0.55.003. Canberra: ABS, 2013. Available at https://www.abs.gov.au/AUSSTATS/abs@.nsf/allprimarymainfeatures/CB60834AB5215E56CA2583530014086D?opendocument

Stephens AS, Purdie S, Yang B and Moore H. Life expectancy estimation in small administrative areas with non-uniform population sizes: application to Australian New South Wales local government areas. BMJ Open 2013; 3: e003710. Available at http://bmjopen.bmj.com/content/3/12/e003710.full