was then derived by producing a ratio o f cause-specific excess risk to the excess risk for all causes. 4. Based on mortality data for Bourke and Brewarrina (see section 4.2 in chapter 4).
Table 5.9 Female cause-specific mortality rates (CSM R)1, relative risk2, and proportion of excess risk3 for Aborigines and non-Aborigines living in two western New South Wales communities4, 1979-1988, as compared with the New South Wales population (NSW )5, 1986. Cause of death Community Deaths CSMR NSW CSMR Relative risk range Proportion of excess risk5 Aboriginal All causes 55 20.3 7.1 (2.2 - 3.8) 100 Malignant neoplasms 6 2.5 1.6 (0 .7 -4 .1 ) 6 Circulatory system 23 10.8 3.7 (1 .9 -4 .6 ) 54 Respiratory system 2 1.0 0.4 - 4 Digestive system 3 1.1 0.3 - 7 Accidents, poisonings and violence 8 1.0 0.3 (1 .7 -7 .8 ) 5 Other causes 13 3.9 0.8 (2.9 - 9.3) 23 Non-Aboriginal All causes 95 5.4 7.1 (0.6 - 0.9) 100 M alignant neoplasms 16 0.9 1.6 (0.3 - 0.9) 46 Circulatory system 54 3.1 3.7 (0 .6 - 1.1) 36 Respiratory system 6 0.3 0.4 (0.4 - 2.4) 3 Digestive system 5 0.3 0.3 (0.5 - 3.7) -3 Accidents, poisonings and violence 4 0.2 0.3 - 7 Other causes 10 0.6 0.8 (0 .4 - 1.6) 11
1. Rates are deaths per 1,000 population per year. Aboriginal and non-Aboriginal crude death rates were directly standardized to the N ew South Wales age structure, 30 June 1986.
2. A ratio o f the directly standardized Aboriginal or non-Aboriginal rate to the total N ew South Wales rate. The relative risk range represents confidence intervals, at a 95 per cent level o f confidence, around each ratio. A relative risk range was calculated for cause-specific mortality rates based on five or more deaths. 3. Excess risk was calculated by subtracting cause-specific death rates for the total N ew South Wales
population from directly standardized Aboriginal and non-Aboriginal rates. The proportion of excess risk was then derived by producing a ratio o f cause-specific excess risk to the excess risk for all causes. 4. Based on mortality data for Bourke and Brewarrina (see section 4.2 in chapter 4).
5.5.2 Indirect estimates of Aboriginal mortality
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An indirect method of estimating adult Aboriginal mortality was used to evaluate the validity of the above direct estimates and to assess the past duration of current patterns of adult survivorship. In this case, adult sex-specific mortality rates were indirectly estimated using the Brass methods of estimating conditional adult survivorship from maternal and paternal orphanhood data (United Nations, 1983: 97-107). These methods require information on the proportion of respondents with a surviving mother or father and on the mean age of mothers or fathers at the birth of their children. Owing to the lack of data on registered births, estimates of the mean age of maternity and paternity were based on values used by Gray (1987a) for Aborigines in northern New South Wales. Adult mortality estimates were produced in the form of survivorship ratios which measure the probability that men aged 25 years and women aged 35 years (the estimated mean ages of paternity and maternity) will still be living at increasing age increments from these ages.
As already stated, the household questionnaire used in the survey collected
demographic details of all persons living in a household, including their sex, age or date of birth, and whether their parents were still alive (see section 5.4.1 and household data sheet 1 in Appendix 2). In total, information on the survival of parents was obtained from a non-random sample of 229 individuals, or 32 per cent of the total Bourke Aboriginal population in 1986.
As shown in Table 5.10, the proportion of Bourke Aborigines with both mother and father living decreases quite rapidly with age; by the time individuals reach 40 years of age or more, it is very unlikely that both or even one parent will be living. For the 20- 29 years age group, 71 per cent have both parents living, 18 per cent have only a mother living, 3 per cent have only a father living, and 8 per cent have neither parent living. This pattem of the survival of parents seems to closely resemble that obtained by the
Aboriginal Family Demography study for Aborigines living on the north coast of New South Wales (Gray, 1987a).
The estimation of adult survivorship from Bourke Aboriginal paternal and maternal orphanhood data was rather more successful for women than for men (see Table 5.11). Survival ratios calculated for Bourke Aboriginal females are strikingly similar to those produced for Aboriginal females living in western New South Wales, 1984-1987. For example, the survivorship of women aged from 25 to 50 years was 0.88 for Bourke and 0.90 for western New South Wales. In fact, only for women aged from 25 to 70 years is there a substantial difference in survivorship estimates. However, this rather low
estimate of survivorship is most probably due to the effects of small numbers.
The close similarity of mortality rates for female Bourke and western New South Wales Aborigines is not surprising, considering that there are very few differences in female life expectancies between these two groups after the age of 25 years. As shown in Table A 1.15 in Appendix 1, differences in life expectancy after age 25 in Bourke and Brewarrina, 1979-1988, and western New South Wales, 1984-1987, were generally not significant. In fact, the critical ratios which were produced suggest that only at ages 25 to 34 years were life expectancies significantly different at a 0.01 probability level. The survivorship ratios produced here for female Bourke Aborigines are not all contemporaneous; rather, they refer to specific periods in time before the survey was conducted. For example, the time estimate for the survival of women from ages 25 to 70 years, produced on the last row and column of Table 5.11, is based on reported deaths which occurred approximately 17 years ago to mothers of persons now between the ages of 40 and 50 years. Overall, these time estimates suggest that the current high mortality rates among female Bourke Aborigines have existed for a relatively long time — at least 17 years.
Page 161 Indirect estimates of male survivorship were on the whole much less satisfactory. In fact, the pattern of male mortality described here is not plausible, because male
survivorship does not decrease with age. The Aboriginal Family Demography study on the north coast of New South Wales had a similar problem of estimating male
survivorship. Gray (1987a: 9-13) attributed this difficulty to a variety of underlying causes including: the quality and reliability of collected data; the intrusion of high adult male mortality into the childbearing years; and the likely difference between the
survivorship of married and unmarried men. These reasons almost certainly were important here as well.
Conclusions which can be drawn from this exercise are certainly not clear-cut. For women, estimates of adult survivorship suggest that the pattern of mortality in Bourke is virtually identical to that in western New South Wales from 1984 to 1987 and that this pattem has existed for at least 17 years. However, no generalizations can be drawn from the estimated male survivorship ratios. Overall, the evidence presented here at least partially supports the idea that high adult Aboriginal mortality is not a new phenomenon, but one that has existed for some time.
Table 5.10 Survival of parents of Bourke Aborigines based on a non-random sample1 of adults aged 18 years and over.
Parents of Bourke Aborigines
Age group Total Neither living Only father living Only mother living Both living Both living (per cent) North coast (per cent)2 0-4 37 0 0 1 36 97 96 5-9 27 0 0 2 25 93 95 10-14 17 0 0 0 17 100 85 15-19 42 1 0 4 37 88 70 20-29 62 5 2 11 44 71 69 30-39 25 4 2 4 15 60 26 40 plus 19 12 1 4 2 11 6 All ages 229 22 5 26 176 77 63