EXPLANATORY NOTES 2 - Estimating interstate migration
Unlike overseas migration, interstate migration in Australia is totally unregulated. It falls within the unwritten freedoms enjoyed by Australians to live wherever they please and to move from one address to another without being accountable to government. In the absence of regulation, however, problems arise when data is required on interstate movements - for example when updating the populations of the States. It has therefore always been necessary to estimate interstate arrivals and departures for this purpose.
The first attempt to compile estimates was made for a period prior to 1966 when net interstate movements were estimated from records of all movements (including short-term movements) by sea, air and rail. From June 1966 to 1981, interstate movements were based on records of interstate changes of address on child endowment (family allowance) registers and electoral rolls. Holiday, business and other short-term movements were no longer taken into account. The change was made in the recognition that estimates based on air, sea and rail travel were inadequate as measures of total net interstate movements (travel by car, for example, was not recorded) and that measuring all movements including short-term moves was neither necessary nor practicable.
At the time, the family allowance was paid universally (to all mothers of children under the age of 17 years) but in November 1987 means testing was introduced. An upper income limit for entitlement was set, graduated according to the number of children in the family.
The Department of Social Security (DSS) was unable to estimate the impact of means testing. In October 1987 there were 2.1 million families collecting the allowance, one month later it declined to 1.8 million. It then started to gradually increase, reaching 1.9 million in August 1989. At the time of introduction of means testing the Department had no data on income of parents. As a result, the department could not exactly explain whether the decline since November 1987 was due to:
· income testing families with the youngest child turning 17 years (and therefore becoming ineligible).
· families with the youngest child turning 17 years (and therefore becoming ineligible)
Whatever the answer, it was clear that means testing, by reducing coverage of the population, reduced the effectiveness of the family allowance register as an indicator of interstate migration. In addition, effectiveness was subject to changes over the years to the age limit of children for which family allowance was applicable.
Electoral roll data were used in conjunction with family allowance data, but were much less reliable as a source of interstate migration statistics. While the registration of electors, voting, and notification of a change of address are compulsory in law, several factors affect the quality of the data. First, a proportion of the population are not entitled to vote and are therefore not on the electoral roll. Second, registration of persons entitled to vote is incomplete. Third, the timing of elections has affected the timing of notification of change of address. Fourth, the timing of Electoral Office surveys to check the coverage of their rolls (called 'habitation' reviews) also affect the timing of notification of change of address.
All these factors affect both the coverage of the data and the reliability of the interstate migration estimates. The electoral data usually showed a much lower
level of residential moves and very large fluctuations reflecting the timing of elections. The quality of electoral transfer data deteriorated after 1976 and by 1981 was no longer considered a reliable source of interstate migration statistics.
Because the combined scope of electoral data and family allowance data did not provide full coverage of the entire population, adjustments were made to allow for the persons not covered, namely persons over 15 years of age but below voting age, and others not eligible to vote.
The deterioration of the electoral transfer data led to their abandonment in 1981. From 1981 to 1986, family allowance data only were used but in October 1987, when the means test for family allowance payments was introduced, it was also replaced.
A detailed assessment of the quality of the family allowance data, the internal migration survey (a former annual survey on change of usual residence) and, the electoral roll transfer data, are presented in an ABS Occasional Paper, Post-censal interstate migration estimates 1966-1981 (O.B. Di Iulio, ABS Occasional Paper No. 1984/2).
As from 1986, transfer (interstate migration) data became available from the nationwide compulsory Medicare system and they were assessed to be of high quality. They have been used since then, in place of family allowance data, to estimate interstate migration.
Estimates of interstate migration based on Medicare data are expected to be higher than census data for two reasons:
· An individual can have multiple moves registered with Medicare over a one year period, while only one move can be identified through the census
· Census data is subject to net undercount of the population during the census.
While Medicare theoretically covers all Australian usual residents as well as those non-Australian residents granted temporary registration, there are a range of Australian usual residents who do not access the Medicare system, primarily due to access to alternative health services (for more information see Demography Working Paper 96/1 Evaluation of Administrative Data Sources for Use in Quarterly Estimation of Interstate Migration Between 1996 and 2001). Such people include some Indigenous persons, defence force personnel, prisoners and persons eligible for Department of Veterans' Affairs Health Services. Furthermore, there are also those individuals who simply do not register their change of address with Medicare when they move, even though they continue to access the Medicare system. As such, Medicare data on interstate movers will have a degree of undercoverage.
Until June 1996, only Medicare transfers aged 1-14 were used, as most other ages suffered from significant under-registration of transfers. These transfers were then expanded with adult to child expansion ratios based on information from the latest available census being applied to Medicare transfer data for ages 1-14 for each interstate flow. However, estimates of interstate migration from July 1996 have used Medicare transfer data for all ages as a result of improvement in registration for all ages. Medicare data for each age is then used to represent total movements for that age, after making adjustments for under-registration by comparing Medicare data with that from the latest available census.
Undercoverage of movers by Medicare is incorporated through the use of expansion factors which scale up Medicare data to reflect the higher levels of Census movers (adjusted for net undercount and multiple movers). This adjustment for undercoverage is applied to Medicare data for males aged from 16 to 29 years and females aged from 18 to 24 years as investigations comparing Census data and Medicare data suggest that undercoverage is most significant for these people.
Details of this method are included in Demography -> Concepts Sources and Methods. A complete overview of interstate migration estimation is given in the Demography Working paper 99/2: Estimating Interstate Migration, 1996-2001.