Alcohol and other drug use - feature analysis

NACS linked dataset

The data set used for these analyses was created by the Australian Institute of Health and Welfare (AIHW) Data Linkage Unit by linking health and welfare data sets held by the AIHW. Approval for this project was provided by the AIHW ethics committee under project number EO2023/2/1413.

The final linked data asset is referred to as the NACS dataset and contains the following source datasets:

• National Death Index (NDI) January 2011 to May 2024.
• Alcohol and Other Drug Treatment Services (AODTS) July 2012 to June 2023.
• Commonwealth primary health datasets
  • Pharmaceutical Benefits Scheme (PBS) January 2007 to February 2023.
  • Medicare Benefits Schedule (MBS) January 2007 to February 2023.
• Specialist Homelessness Services Collection (SHSC) July 2011 to June 2023.

Data linkage

To create the study cohort, identifiers from SHSC, AODTS and NDI were linked to AIHW Enhanced Medicare Spine (EMS). The EMS includes all individuals who have registered with Medicare since 1984, based on Medicare enrolment data.

The cohort was formed as a combination of all individuals (linked and unlinked) from SHSC, AODTS and NDI datasets for the period from 01/01/2012 to 31/12/2022 (or latest available data) and consisted of SLK-581s (statistical linkage key 581). The SLK-581 is a concatenation of s235 (2nd, 3rd and 5th letters of surname), f23 (2nd and 3rd letters of first name), date of birth and sex. SLKs are used to preserve privacy in record keeping and during linkage. In general, the uniqueness of SLK-581 keys for a given Australian population is high.

For the SHSC and the AODTS, an initial stage of simple merging by common fields was undertaken. First, all unique matches by SLK and postcode were obtained, followed by all unique matches by SLK and jurisdiction. This stage was followed by a ‘dropout-1 linkage’ to complete the results. The ‘dropout-1 linkage’ has been found to be an effective approach to linkages using the statistical linkage key SLK-581 together with postcode. This approach allows for disagreement of a single SLK component or postcode in potential record pairs found multiple deterministic passes.

Linkage results – Alcohol and Other Drug Treatment Services National Minimum Data Set (AODTS NMDS)

A total of 703,847 records were linked from the Alcohol and Other Drug Treatment Services National Minimum Data Set, which accounts for 82.3% of the 855,559 cohort SLKs. Previous experience with linking AODTS NMDS data to spine versions containing Medicare and DOMINO identifiers suggests that a significant portion of the remainder is likely composed of alternative SLKs for cohort members who have otherwise already found EMS matches but were not able to be linked without the benefit of DOMINO data. Despite this, since around 1 in 5 records could not be matched, the number of people who died and had received specialist AOD treatment services is likely to be an underestimate of the true number of deaths.

There were no notable differences in linkage rates between males and females, or for different age groups. Unlinked records had a larger number of missing identifier fields and were more likely to have dummy day, month and year of birth, which are permitted within the AODTS NMDS.

There is some variation in the linkage rates across time (Table Tech1), as expected. This is largely expected due to both an overall improvement in SLK quality over years, as well as quality issues due to system changes in various states at different time points.

Note: Records in this table refer to a unique combination of SLK/ year/ state/ postcode/ suburb using client geography.

National Death Index

The National Death Index (NDI) data provided for this project included all records, including those that were not linked to the EMS. Data cleaning was undertaken to remove potential duplicate records and any records that did not have a cause of death.

In the version of the NDI database used in the NACS, data were considered final for those deaths registered prior to 2021, revised for deaths registered in 2021 and preliminary for deaths registered in 2022 and 2023. Preliminary data are subject to further revision. Data for 2018–2022 reflect the updated ABS enhancements applied from 2024, as outlined in the Technical notes.

Medicare Benefits Schedule

Medicare Benefits Schedule (MBS) data for the 5 years preceding the date of death was included for all people who died. MBS data was not included for people who were not linked to the NDI. Data from January 2017 to February 2023 were included in the final dataset.

Pharmaceutical Benefits Scheme

Pharmaceuticals Benefits Scheme (PBS) data for the 5 years preceding the date of death was included for all people who died. PBS data was not included for people who were not linked to the NDI. Data from January 2017 to February 2023 were included in the final dataset. Technical Note