Australian Bureau of Statistics (ABS) changes to mortality coding over the study period

The following information on mortality coding is sourced from the ABS. For further information, see the ABS Causes of death, Australia report.

Substantial changes to ABS cause of death coding were undertaken in 2006, improving data quality by enabling the revision of cause of death for open coroner’s cases over time. Deaths that are referred to a coroner (including deaths due to suicide) can take time to be fully investigated. To account for this, all coroner-certified deaths registered after 1 January 2006 are subject to a revisions process. This allows cause of death for open coroner’s cases to be included at a later stage where the case is closed during the revision period. Cause of death data are deemed preliminary when first published, with revised and final versions of the data being historically published 12 and 24 months after initial processing. Prior to 2006, revisions did not take place and as such it is recognised by the ABS that suicide deaths may have been understated during this period (ABS 2018).

As well as the above changes, new coding guidelines were applied to deaths registered from 1 January 2007. The new guidelines improve data quality by enabling deaths to be coded as suicide by ABS mortality coders if evidence from police reports, toxicology reports, autopsy reports and coroners’ findings indicates the death was due to suicide. Previously, coding rules required a coroner to determine a death as due to suicide for it to be coded as suicide.

The combined result of both changes has been the more complete capture of suicide deaths, and a reduced number of deaths coded as ‘undetermined intent’, within Australian mortality data.

Detailed information on coding guidelines for intentional self-harm, and administrative and system changes that can have an impact on the mortality data set, can be found in Explanatory Notes 91-100 of Causes of death, Australia report (ABS 2018).

Changes to previously published suicide information

As well as the addition of a new year of cause of death data, there are three main reasons for changes to previously published suicide results, as described below.

Lag in cause of death information for the most recent year of data, where a death is registered in the following year

Analysis in this study is based on year of occurrence of death. The National Death Index (NDI) is the source of information on fact of death in this study. Fact of death information from the NDI is supplemented with cause of death information from the National Mortality Database (NMD). Results published in the report National suicide monitoring of serving and ex-serving Australian Defence Force personnel: 2018 update for deaths that occurred in 2016 were based on preliminary cause of death information from the NMD. This was the most recent version of cause of death information at the time of reporting.

Analysis of the NMD for all Australian deaths shows that between 4% and 7% of deaths are not registered until the next year (ABS 2018). These deaths are not captured in cause of death information, until data for the next year become available. This means that while fact of death information was complete for 2016 at the time of publishing the National suicide monitoring of serving and ex-serving Australian Defence Force personnel: 2018 update, cause of death information was missing for around 7.5% of the deaths included in the analysis at that time. Additional suicides that occurred in 2016 but that were not registered until 2017 have now been identified with the inclusion of preliminary 2017 cause of death information in the current results.

Cause of death data revisions (ABS)

Cause of death information for the National suicide monitoring of serving and ex-serving ADF personnel: 2019 update release is based on final cause of death information for the years 2001 to 2014. Revised data are used for 2015 and preliminary data for 2016 and 2017. Cause of death for a small number of records linked to the 2015 (revised), 2016 (preliminary) and 2017 (preliminary) cause of death data may change where a death is being investigated by a Coroner and more up to date information becomes available as a result of the ABS revisions process. This may have a small effect on the number of deaths attributed to suicide in these years, as some deaths currently coded as ‘undetermined intent’ could later be identified as ‘intentional self-harm’.

Improvements in information available to the study

Changes to previously published results may also occur as additional information becomes available to the study.

For example, changes affecting recording of deaths in jurisdictional systems (including administrative and system changes, certification practices, classification updates or coding rule changes) can impact on the data sets underlying this study. Data users should note the potential impact of these changes when making comparisons between reference periods. While such changes will not explain all differences between years, they are a factor that may influence the magnitude of any changes in suicide numbers as revisions are applied (ABS 2018)

Rates based on small numbers

Rates based on small numbers of events can fluctuate from year to year for reasons other than a true change in the underlying frequency of the event.

In this report, rates are not reported when there are fewer than 5 events, as rates produced using small numbers can be sensitive to small changes in counts of deaths over time.

Suicide rates

When reporting how often suicide occurs, suicide rates account for the size of the underlying population. Whilst this measure does not account for differences in the age structures of the populations being compared, rates can be validly compared over time, across groups and to the corresponding Australian population.

Here, rates report how often suicides occur in the three ADF service status groups, and Australia, expressed as a number per 100,000 people.

Standardised mortality ratios

The standardised mortality ratio (SMR) is a widely recognised measure used to account for differences in age structures when comparing death rates between populations. This method of standardisation can be used when analysing relatively rare events (i.e. where number of deaths is less than 25 for the analysed time period) (Curtin and Klein, 1995). The SMR is used to control for the fact that the three ADF service status groups have a younger age profile than the Australian population, and rates of suicide vary by age in both the study populations and the Australian population. The SMRs control for these differences, enabling comparisons of suicide counts between the three service status groups and Australia without the confounding effect of differences in age.

The SMR is calculated as the observed number of events (suicide deaths) in the study population divided by the number of events that would be expected if the study population had the same age and sex specific rates as the as the comparison population.

SMRs greater than 1.0 indicate a greater number of suicides in the ADF population than expected; and SMRs less than 1.0 indicate a lower number of suicides than expected in the ADF population.

Using confidence intervals to test for statistical significance

Statistical significance is a measure that indicates how likely the observed difference is due to chance alone.

In this study, 95% confidence intervals (CIs) are provided for each standardised mortality ratio (SMR) and suicide rates to indicate the level of uncertainty around these estimates. Estimates produced using low numbers can be sensitive to small changes in numbers of deaths over time and will therefore have wide CIs. 95% CIs are provided within this report as they may account for the variation in absolute numbers of suicide deaths over time (related to the small sample size).

Use of CIs is the simplest way to test for significant differences. For the purpose of this report, differences are deemed to be statistically significant if CIs do not overlap with each other (when comparing suicide rates) or 1.0 (in the case of an SMR).

Where the CIs are wide, for example in the case of the SMR for ex-serving females, sensitivity analysis was conducted. This analysis found that slight changes to the numbers of suicides did not significantly alter the result.

Acronyms

ADF

Australian Defence Force

AIHW

Australian Institute of Health and Welfare

CI

confidence interval

DSD

Defence Suicide Database

DVA

Department of Veterans’ Affairs

NDI

National Death Index

NMD

National Mortality Database

PMKeyS

Personnel Management Key Solution

SMR

Standardised mortality ratio