methods
Measures
Mortality rates, and disease incidence and prevalence rates, are standard epidemiological measures of the extent of disease. In addition to these measures, the 2000 SA Burden of Disease Study makes use of a summary measure called disability-adjusted life years (DALY). DALY comprises both years of life lost due to premature mortality (YLL), and years lived with a disability, weighted according to the severity of the disability (YLD). In effect, DALY uses time to equate death and disability.
Age weighting, time discounting of 3% per annum and the standard life expectancies based on the West model levels 25 and 26 (considered to be a maximum life expectancy) are used to calculate the years of life lost to premature mortality. This is the standard set by Global Burden of Disease project of the WHO.
Age-standardised mortality rates were calculated to enable comparison of the levels of mortality experienced in each province using the WHO world standard.
Disease categories
An adapted version of the 1990 Global Burden of Disease list of causes of death was developed for the 2000 SA Burden of Disease Study.
Overall mortality is divided into three broad groups of causes:
Group I |
Pre-transitional causes. Includes communicable diseases, maternal causes, perinatal conditions and nutritional deficiencies. HIV/AIDS is part of Group I but kept separate in this analysis due to the size of the burden it contributes |
Group II |
Non-communicable causes |
Group III |
Injuries |
Each group is divided into several major categories of causes of death, such as respiratory infections, cardiovascular diseases and intentional injuries. These categories are further disaggregated into specific diseases or disease clusters.
Mortality estimates
The number of deaths for 2000 was estimated using the ASSA2000 model of the Actuarial Society of SA, calibrated according to estimates of child and adult mortality based on surveys, census results and vital statistics. The model is also calibrated to replicate antenatal HIV seroprevalence survey data for pregnant women who attend public sector clinics.
The non-AIDS cause of death profile was estimated from three sources. Firstly, 1996 cause of death data from Statistics South Africa were used to provide information on the profile for Groups I and II, excluding AIDS. Ill-defined causes within a disease category were reallocated proportionally by age and sex to specified causes within that category. Experts were consulted to refine this reallocation, particularly for cardiovascular diseases, perinatal conditions and congenital abnormalities. Secondly, cause of death information processed by the Department of Home Affairs was used to estimate the overall proportion of deaths due to injuries by age and sex. Thirdly, the UNISA/MRC national injury mortality surveillance system (NIMSS) was used to estimate the profile of causes of deaths resulting from injury.
In the case of provincial estimates, the cause of death profile based on Statistics South Africa’s 15% sample was also used.
Non-fatal outcome estimates
The extent of illness and disability was estimated based on the ratios of years lived with disability (YLDs) to years of life lost (YLLs) for each of the disease categories, as estimated by the World Health Organisation (WHO) for the Afro E region, of which SA is a part. This was done for illustrative purposes only.
Revised DALY estimates
As more data have become available, it has been necessary to update and refine the estimates of the burden of disease. The risk factor study makes use of a second revision of DALYs for South Africa for the year 2000. ...read more.
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