Risk factor attributable burden

Overarching methods and choices for risk factors

Most of the risk factors methods used in the Australian Burden of Disease Study (ABDS) 2024 were the same as those used in the ABDS 2018 (AIHW 2021a). General methods and choices for risk factors can be found in Overarching methods and choices for risk factors and Risk factor attributable burden (AIHW 2021b). This includes descriptions of the methods used to calculate the population attributable fractions (PAFs) and attributable burden, including the selection of linked diseases, estimation of effect sizes (relative risks), combined risk factor analysis and assumptions for theoretical minimum risk exposure distributions (TMREDs).

The basic steps for estimating attributable burden are described as follows:

  • Select linked diseases for which there is convincing or probable evidence in the literature that the risk factor has a causal association.
  • Define the exposure to the risk factor that is not associated with increased risk of the linked disease (the theoretical minimum risk exposure distribution or TMRED).
  • Estimate the PAFs by either the comparative risk assessment method or the direct method:
    • Comparative risk assessment involves using the amount of increased risk (relative risk) of linked disease morbidity or mortality due to exposure to the risk factor and an estimate of exposure to each risk factor in the population. For most risk factors, exposure to the risk factor was estimated using high-quality survey data. For information about the quality of data inputs, see Australian Burden of Disease Study: Methods and supplementary material 2018.
    • The direct method uses comprehensive data sources such as registries to estimate the amount of the linked disease due to the risk factor.
    • Estimate the attributable burden by multiplying the PAFs by the disease burden (fatal and non-fatal) for each linked disease.

The linked diseases and relative risks were sourced from the GBD 2019 or an AIHW review of the literature. Most TMREDs were also sourced from the GBD 2019, with the exceptions described in Risk factor specific methods.

Exposure to risk factors in the lifetime of the individuals in the population can influence the proportion of burden in the reference year. For risk factors such as tobacco use, occupational risks, alcohol use, child abuse and neglect, illicit drug use, and unsafe sex, the burden can continue to exist from past exposure levels. Where evidence of ever being exposed to a risk factor can be linked to current burden, this is included in the analyses and described under the individual risk factor.

For some risk factors, such as overweight (including obesity), current exposure can have an impact on future burden. This is not accounted for in this study as the burden pertains to each reference year (2003, 2011, 2015, 2018 or 2024).

Not all risk factors are relevant to all population (age and sex) groups. For example, the bulk of the burden from high blood pressure occurs for people aged 25 and over. The choices for population groups and type of burden (fatal or non-fatal) were informed by the GBD 2019 (GBD 2019 Risk Factor Collaborators 2020). The population group for which attributable burden from a given risk factor has been estimated is described in each section.

Both fatal and non-fatal burden are relevant for most linked diseases in the study. For others, such as back pain & problems linked to occupational risks, only non-fatal burden has been estimated.

Note that for the majority of the analysis in this report, the burden from different risk factors for a particular disease cannot simply be added together, because:

  • some risk factors are on the same causal pathway – for example, a diet high in sodium increases the likelihood of high blood pressure
  • the PAFs are estimated independently – the burden due to each risk factor for a given disease might exceed the total burden of that disease.

Combined risk factor analysis was undertaken to measure the combined effect of multiple risk factors and account for the bias introduced by the complex pathways and interactions between many risk factors.

Firstly, to account for risk factors on the same causal pathway, mediation factors were used to attenuate the relative risk for the first risk factor in the pathway which mediates through the second risk factor in the same causal pathway for the relevant linked disease. The attenuation factors were sourced from the GBD 2019 (GBD 2019 Risk Factors Collaborators 2020).

Following mediation, the combined burden of more than 1 risk factor was adjusted to prevent the combined disease burden exceeding the total burden for a given disease (the ‘joint effect’).

The use of both the joint effect and mediation formulae therefore adjusts for the interrelatedness between risk factors in the same causal pathway as well as the combined impact of all risk factors and all dietary risks included in the study. Detailed examples of this approach, also used for ABDS 2018, are further described in Risk factor attributable burden (AIHW 2021b).

A supplementary table contains detailed definitions, data sources and linked diseases for all risk factors .

Calculating attributable deaths for 2024

Attributable deaths provide an estimate of the number of deaths attributable to each risk factor. Attributable deaths are estimated in the same way that disease burden attributable to risk factors is calculated, by applying estimated fatal PAFs to the redistributed number of deaths for that year.

An estimate of attributable deaths is not provided for 2024, as data on deaths in 2024 were not available at the time of analysis. However, an approximate percentage of attributable deaths for 2024 is provided in order to provide some information on attributable deaths in 2024. The percentage of attributable deaths is considered to be less sensitive to unpredictable fluctuations in deaths that occur over time. The percentage of attributable deaths for 2024 were estimated based on the projected YLL in 2024 divided by the mean remaining life expectancy for each age group. 

Where attributable deaths are reported (for 2003, 2011, 2015 and 2018), attributable deaths are based on deaths that have been redistributed for fatal burden analysis. As such the number of deaths may not align with other reporting of causes of death. Information on the redistribution of deaths can be found in the Australian Burden of Disease Study: methods and supplementary material 2018 report. 

Nowcasting population attributable fractions

For the first time, ABDS 2024 used nowcasting to project estimates of population attributable fractions (PAFs) where possible using available data. These are applied to burden of disease estimates, where nowcasting has also been used to project estimates of disease burden to the current year based on available data.

For ABDS 2024, PAFs were nowcast to the current year of 2024 using a beta regression model. Beta regression was chosen as it can be used to model proportion data, making it an appropriate choice for nowcasting PAFs.

The nowcast model is based on trends in PAFs estimated for earlier reference years, as well as PAFs based on the latest available exposure data. PAFs were nowcast by each sex, age group, risk factor and disease/injury group.

For example, updated body mass index (BMI) data from the National Health Survey (NHS) 2022 was used to estimate a new 2022 PAF for the overweight (including obesity) risk factor. This 2022 estimate was incorporated alongside earlier estimates from 2003, 2011, 2015 and 2018 to nowcast a PAF for 2024.

The ability to nowcast PAFs was assessed on a case-by-case basis. The risk factors with PAFs in-scope for nowcasting include: 

  • Overweight and obesity
  • High blood pressure.

Nowcasting PAFs was not possible or considered necessary for risk factors where:

  • The latest exposure data is considered up to date, with minimal benefit from nowcasting, such as alcohol use based on National Drug Strategy Household Survey (NDSHS) data 2022–2023.
  • There is no trend information available or the same PAF for a risk factor is applied to all ABDS years, such as for bullying victimisation and child abuse and neglect.
  • PAFs are stable and there will be little benefit gained from nowcasting.
  • PAFs are volatile and subject to unpredictable changes, as for environmental risk factors such as air pollution.

Where there was no new trend data available, PAFs from ABDS 2018 were carried forward to ABDS 2024.

Attributable burden data quality

Survey and administrative data sets were primary sources of risk factor exposure data. In the absence of good-quality survey or administrative data, epidemiological studies were used to determine exposure distributions.

The quality of input estimates in the ABDS 2024 for earlier reference years (2003, 2011, 2015 and 2018) are generally the same as the quality presented in the ABDS 2018. Refer to Appendix B in the Australian Burden of Disease Study: impact and causes of illness and death in Australia 2018 report (AIHW 2021a) and the Australian Burden of Disease Study: methods and supplementary material 2018 report (AIHW 2021b) for more detail on the quality of the risk factor exposure data, including details on the criteria used to assess risk factor exposure data selection.

Data sources that were changed in ABDS 2024 (such as the epidemiological study used to estimate attributable burden due to UV sun exposure) are described in detail below and the quality is expected to be similar to ABDS 2018.

Risk factor specific methods

This section describes in detail the methods unique to each risk factor included in the ABDS 2024. It is focused on the calculation of exposure estimates, as this was the aspect of risk estimation most influenced by Australia-specific data, and methods used to nowcast estimates to the 2024 reference year.

Behavioural risk factors

Metabolic/biomedical risk factors

Environmental risk factors