2021 Census update of the Net Interstate Migration model

Quarterly estimates of interstate migration are published in National, state and territory population.

The main input to interstate estimates is Medicare change of address information supplied by Services Australia.

The Medicare system covers:

• Australian citizens
• permanent residents
• certain temporary visa holders.

The Medicare system excludes people on international student visas or temporary work visas (482).

Some people, particularly younger adults, do not register changes of address with Medicare or do so long after they move. This means that the Medicare data underestimates interstate migration for certain age groups.

Comparing Census and Medicare data over time shows that the level of under-reporting in Medicare changed between 2016-2021.

• This coincides with the uptake of COVID-19 vaccinations in 2021 across several states and territories, which changed usual Medicare reporting behaviour.
• Young adult males are more likely to be under-represented in the Medicare address change data, and across states and territories.
• Additionally, there was an increase in electronic Medicare claiming options at the doctor's office, online and by mobile phone app. This increased under-reporting of address change. Under-reporting was previously confined mainly to young adults aged under 30, but over time has increasingly affected older ages. 

The below graph shows the number of people who reported in the Census that their address one year ago was interstate from their current address, divided by the number of address changes supplied to Medicare for the same year. A ratio higher than one suggests that more people reported in the Census that they had moved interstate than had reported their move to Medicare. 

Despite these limitations, Medicare data is the most effective source of quarterly internal migration currently available, based on timeliness and population scope. To address some of these known limitations, we also take Defence force movements and Census 1 year ago data to supplement Medicare.

Australian defence force personnel have access to alternative health services and so may not use Medicare's services. To account for this, 70% of interstate movements by defence force personnel (calculated by age, sex and state/territory of arrival and departure) are added to the Medicare data. 

It is not known what proportion of defence personnel opt to use Medicare instead of Defence's health system. The 70% factor is an estimate based on the assumption that single people are most likely to exclusively use the Defence health service whereas personnel with a partner or dependents are likely to be listed on the same Medicare card as their family members and so captured in the Medicare address change data.

The Australian Census includes a question on address of usual residence one year ago and address five years ago.

• Census one year ago is used as an alternative interstate migration estimate and an input into expansion factors.
• Census five years ago is used for validation of results.

These Census-based estimates complement the Medicare-based estimates.

• Census provides a one-year and five-year snapshot of interstate migration.
• Quarterly estimates from Medicare are needed to estimate resident population on a state and sub-state level.

There are some limitations to Census data.

• The 2021 Census was run during COVID-lockdowns in some states and territories, which means the data reflects a unique COVID-affected pattern.
• The Census does not capture multiple interstate movements by the same person within the one or five-year period.
• Non-response - either where no Census form was received from an individual, or the questions are not answered. 

The 2021 Census had a non-response of 5.9 % to the question on place of address one year ago.

There was little variation in the response rate between sexes or between states (other than Northern Territory, 9.8%).

The non-response rate did vary by age, with people over age 75 more likely not to answer.

The biggest conceptual difference in coverage between Census data and Medicare data is their ability to capture multiple interstate moves made by the same person within the year.

• Medicare records up to four moves per year (one per quarter).
• Census records a maximum of one move for the year.

To make the two data sources more comparable, Census data is inflated by the percentage of movements captured by Medicare which are not covered by Census data.

This exercise is to cover the following scenarios:

• If a person moves interstate (for example, from New South Wales to the Australian Capital Territory) and then in a later quarter of the same year moves to a third state (for example, from the Australian Capital Territory to Victoria), Medicare records two moves, but Census only records one (New South Wales to Victoria).
• If a person moves interstate (for example, from New South Wales to the Australian Capital Territory) and then returns to their original state within the same year (for example, from the Australian Capital Territory to New South Wales), Medicare will record two moves, but Census will record no move at all.

To estimate how many records had multiple interstate movements by the same person, we calculate a multiple mover factor.

To calculate the multiple mover factor, we matched anonymous records from the quarterly Medicare data by:

• age
• date of birth
• enrolment type
• postcode.

We then applied the percentage results to the Census data by single year of age/sex/state.

In 2021 the multiple mover percentage was 7.5%.

Census estimates include defence force personnel, so an adjustment is made to remove these extra movements. This is to allow for a direct comparison with Medicare movements, which does not cover defence force movers.

This adjustment is for: 

• people who moved interstate but returned to their original state, and
• the intermediate movements of a multiple interstate move (e.g. from New South Wales to the Australian Capital Territory to Victoria).

An example to illustrate this: 

• If a person moves interstate and then returns to their original state, Census records no move, so these movements were removed from the defence data before subtracting from Census.
• If a person moves (for example) from New South Wales to the Australian Capital Territory and then from the Australian Capital Territory to Victoria, Census records this as New South Wales to Victoria, so the arrival to and departure from the Australian Capital Territory are missed and therefore were removed from the defence data before subtracting from Census.

Smoothing and capping are both treatments which help reduce volatility in the expansion factors. This is because the expansion factors are based on a comparison of only one year of data, and are limited in their ability to represent a longer period.

During an intercensal period, the relationship between Census and Medicare for a given age/sex/state can fluctuate as actual migration behaviour changes. 

In the past, expansion factors use the most recent year of data as inputs to create these factors. 

In this rebasing, an average of four years (2016-2020) was taken to reduce the impact of COVID-19 affected data inputs for the 2021 expansion factors.

All inputs, as well as the expansion factors themselves, were smoothed by taking a three-term moving average across single years of age.

For example, the smoothed figure for age 24 is the average of the figures for ages 23, 24 and 25. Doing this:

• reduces the impact on future estimates of random noise within the historical data.
• addresses the fact that Census data gives age at the end of the period, rather than age at move.

Smoothing was not applied where this would change the real pattern of the data - i.e. defence was only smoothed starting at age 19, because smoothing ages 17 and 18 distorted the real pattern.

• Expansion factors were capped at 2.0, as has been the practice in the past.
• Capping the expansion factors at 2.0 limits the influence of any one age group, whose behaviour may change over time.

As can be seen in the graph 'Interstate moves, ratio of Census to Medicare-based, by age' above:

• the relationship between Census and Medicare diverges as the number of ages under or over represented increased over the last 15 years
• In 2021, 25% of all ages had expansion factors greater than 1.0, not necessarily consecutively

The possibility of applying all expansion factors greater than 1.0 across all ages were not considered in this final rebasing period as it would assume too much about movement patterns that can change in the next five years.

To reduce this variability and volatility in the model as much as possible, the age range is limited to address the bulk of the under-reporting at the Australia level. 

The age ranges for 2021 final rebasing are: 

• 16-35 years for males
• 16-32 years for females

According to the 2021 Census, this covers:

• 91% of underreporting for males
• 96% of underreporting for females

The method for revising interstate migration estimates is reviewed and adjusted each Census cycle in order to produce the best final estimates.

Two key data sources for interstate migration were affected by the COVID-19 pandemic:

• Medicare data was affected by change of address updating behaviour as people updated their details for Australia's COVID-19 vaccination program
• Place of usual residence one year ago data from the Census was affected by COVID-19 lockdown policies

The response to these data challenges: 

• We changed some data inputs for expansion factors to reduce the effect of COVID-19 affected data
• Instead of taking the most recent year of data from the intercensal period, an average of 2016-2020 financial years for Census-based estimates of NIM, a multiple movers factor, defence and Medicare was used. Using the most recent year of data gave implausible results and could not be taken forward for the next five years
• Census-based estimates is the first data input for expansion factors and is produced by giving weights to Census data. For this rebasing, we gave a weight of 80% to 2016 Census data and a weight of 20% to 2021 Census data. This is different to the 2016 final rebasing where an equal weight was given to the two most recent Censuses.
• Estimates for the September 2020 to June 2021 quarters were revised using Census data instead of expansion factors. This is to keep real-world effects in a year affected by COVID-19.

Final estimates were compared with Census, linked administrative data assets and the change in intercensal difference. Intercensal difference is used as one indicator of the accuracy of interstate migration estimates with more detail in the final rebasing paper Methodology used in final rebased population estimates, June 2021.

Previous paper: 2016 Census Update of the Net Interstate Migration Model, 2011-2016.