*2.1. Data Sources*

Statistics on private health insurance industry activity in Australia are available from the Australian Prudential Regulation Authority (APRA) [13]. For the present study, we extracted quarterly data on the number and total cost of services for each manual therapy profession (i.e., chiropractic, osteopathy, and physiotherapy) from the first quarter (Q1) of 2015 to the second quarter (Q2) of 2020. We also extracted quarterly data on the estimated number of persons covered under private health insurance general treatment cover. A pyramid plot of the number of persons insured by sex and age group in 2020 Q2 is provided in Section A of the Supplementary Materials (Figure S1). Statistics on registered healthcare practitioners are available from the Australian Health Practitioner Regulation Agency (AHPRA) and each profession's registration board [8–10,14]. For this study, we extracted quarterly data on the number of registered providers from each manual therapy profession from 2015 Q1 to 2020 Q2. We did not include registrants listed as limited or non-practicing. We then estimated the number of providers working in the private sector for each profession as follows: For chiropractic and osteopathy, 100% of registrants were taken to be working in the private sector because neither of these professions contribute to service provision in the public sector. For physiotherapy, 63.5% of registrants were taken to be working in the private sector, as estimated by a National Health Workforce Report [15].

#### *2.2. Operational Definitions*

There are three types of private health insurance coverage in Australia: hospital treatment only, general treatment only, and combined hospital and general treatment. General treatment cover includes most services for preventing or managing injuries, diseases, and conditions that are provided outside of the hospital setting. However, it excludes services for chronic disease management that are covered by Medicare. A service was defined as one visit to a healthcare provider.
