Based on Australian Burden of Disease Study 2011, Australian Institute of Health and Welfare. Prepared by the Centre for Epidemiology and Evidence, NSW Ministry of Health.
Years of life lost (YLL): A measure of the years of life lost due to premature death, defined as dying before the average life expectancy for the population as a whole. YLL represent the fatal component of the total burden of disease in a population.
Years lived with disability (YLD): A measure of the years of what could have been a healthy life but were instead spent in states of less than full health. YLD represent the non-fatal burden of disease in a population.
Disability-adjusted life years (DALY): A measure (in years) of healthy life lost, either through premature death (defined as dying before the average life expectancy for the population) (YLL) or, equivalently, through living with ill health due to illness or injury (YLD). DALY represent the total disease burden in a population and is the sum of YLL+YLD.
Health-adjusted life expectancy (HALE): Average life expectancy at full health, or equivalently, average life expectancy free of disability. For each disease group, HALE is interpreted as the average life expectancy free of that disease type.
Please see Australian Burden of Disease Study: impact and causes of illness and death in Australia 2011, supplementary data tables 5a.
Burden of disease analysis measures the combined impact in the entire population of living with illness and injury (non-fatal burden) and dying prematurely (fatal burden). More than merely counting deaths and disease prevalence, it takes into account age at death and severity of disease. The summary measure 'disability-adjusted life years' (or DALY) is used to count the total years of healthy life lost from death and illness in the whole population.
Health-adjusted life expectancy (HALE) is life expectancy adjusted for disease and disability. This is the average number of years that a person can expect to live in “full health” by taking into account years lived in less than full health due to disease and/or injury. For each disease group, HALE is interpreted as the average life expectancy free of that disease type. Note that some diseases with higher case fatality rates (ie a higher death rate following initial diagnosis) may have a lower HALE than chronic diseases (with lower death rates following initial diagnosis).
Information on the health impacts and distribution of different diseases and injuries is important for monitoring population health and in providing an evidence base to inform health-policy and service planning. Burden of disease information can also be used to measure the health impact of interventions, and to highlight which diseases to focus on when investigating the cost-effectiveness of programs and interventions.
The Australian Burden of Disease Study 2011 uses and adapts the methods of global studies to produce estimates that are more relevant to the Australian health policy context. The indicators presented here relate to the NSW population for 2011.
Estimating the fatal burden (YLL)
The first step to estimate YLL is to compile the total number of deaths by age and disease (cause of death). All deaths data used in ABDS 2011 were extracted from the AIHW’s National Mortality Database. This is a register of all deaths in Australia since 1964, sourced from the cause of death unit record files created by the ABS.
YLL for each disease is then calculated at the disease-specific level (for each age). Using single year of age at death, each death is weighted according to the remaining or potential life expectancy at that age of death using the reference life table – this becomes the years of life lost. The ABDS 2011 uses the standard reference life table used in GBD 2010 when calculating YLL for the Australian, subnational and Indigenous populations.
Estimating the non-fatal burden (YLD)
The first step to estimate YLD is to define the major disabling sequelae associated with each disease in the disease list, and attribute disability weights that express the health loss on a scale from 0 (no health loss) to 1 (total health loss) associated with each sequelae. Several sequelae may be associated with each disease. To provide a set of weights for these sequelae, ABDS 2011 has followed the practice of using estimates of the health losses associated with a smaller set of health states to which each of the sequelae can be mapped. The health states and disability weights used in this study for both national and Indigenous estimates are drawn from GBD 2013.
The next step is to compile estimates of point prevalence for each sequela-health state combinations. An adjustment must then be made for the potentially biasing effect of comorbidity; in ABDS 2011, the bias adjustment has been effected through altering the suite of disability weights.
The final step is to calculate YLD for each disease, which is calculated as the point prevalence multiplied by the disability weight summed over all sequela levels (for each age and sex).
Estimating the total burden (DALY)
As they use time as a common currency, the YLL and YLD can be summed to measure DALY: 1 DALY represents the loss of 1 year of healthy life. So, DALY = YLL + YLD.
Further technical details on the computation of YLL, YLD and DALY can be found in the AIHW report Australian Burden of Disease Study 2011: methods and supplementary material.
Estimating health-adjusted life expectancy (HALE)
Using Sullivan’s method, HALE is derived by computing the total years lived without disability using a measure of the years lived at each age group (obtained from ABS life tables for 2010-2012) and a measure of the years lived with disability (obtained from ABDS 2011). Technical details for the computation of HALE can be found in the relevant HealthStats methods paper.