Denominator Matters: Comparing the Impact of Estimated Resident Population Versus Medicare Enrolment Population on...

The Australian Bureau of Statistics (ABS) Estimated Resident Population (ERP) and the Medicare Enrolment population are commonly used denominators in calculating healthcare utilisation rates. The ERP is an estimate of all usual residents of Australia, even those ineligible for Medicare, while the Medicare population is limited to those eligible for Medicare. However, many researchers may be unaware of these differences, which can lead to inappropriate denominator use, misinterpretation of utilisation rates and suboptimal resource allocation. This analysis compared differences in utilisation rates based on whether ERP or Medicare populations were used as denominators.

We compared General Practitioner (GP) utilisation rates across age groups, sex and jurisdictions in Australia. Data on GP attendance (Medicare item 23) and published per capita utilisation rates were obtained from the Medicare Statistics website. Per capita service use rates were recalculated using the ERP at June 2022 as the denominator, with the published number of claims as the numerator. The study analysed data from the financial year 2021-2022.

The ERP included 26 million people, while the Medicare population was 26.2 million. Nationally, GP attendance rates were about 2.3% higher using the ABS ERP than using the Medicare population. However, discrepancies varied by age and jurisdiction. In the ACT, ERP-based rates were around 16% lower than Medicare for persons aged 15-24, whereas in the NT, ERP-based rates for females aged 85+ were 21% higher than Medicare rates.

Nationally, differences between healthcare utilisation rates calculated using ABS ERP versus Medicare enrolment data were minimal. ERP-based rates were lower for younger adults, while Medicare-based rates were lower for older adults. SO WHAT?: Our findings emphasise the importance of carefully selecting and reporting denominators, considering their relevance to the population targeted by the service to ensure meaningful interpretation.