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4.31. Comprehensive statistical records on overseas arrivals and departures have been maintained since the colonial days. This has been made possible by the relative isolation of Australia, the absence of direct land links with other countries, and the limited number of ports of entry. These natural advantages have been reinforced by government control of arrivals and departures.
4.38. There are five possible causes of error in the overseas migration data which can contribute to inaccuracies in population estimates:
Mis-statement of the State of usual residence by permanent and long-term departures
4.39. The extent of this type of error is believed to be small because such people are, or have been permanent residents and can therefore be expected to report their address correctly.
Mis-statement of State of intended residence by permanent and long-term arrivals
4.40. The magnitude of this type of error is illustrated in Table 4.8 which compares the distribution by State of permanent and long-term arrivals for 1980-81, 1985-86, 1990-91 and 1995-96 with the census distribution of Australian residents whose usual residence one year ago was overseas.
4.41. Apart from the disparity between the definition of permanent and long-term residence and 'usual residence' in the Census, the only differences which may affect comparability between these two data sources arise from:
4.42. Notwithstanding these differences, the comparison in Table 4.8 illustrates important differences between the two distributions. Overseas migration statistics show New South Wales and Victoria having a larger share of permanent and long-term arrivals than is shown in census data, while most other States have a smaller proportion than that shown by the census. Queensland in particular shows a significantly higher proportion of permanent and long-term arrivals in the census as compared with passenger card data. This indicates that there is a proportion of migrants who record New South Wales and Victoria as their State of arrival on the passenger card, but move interstate during their first year in Australia.
4.43. These differences between overseas migration data and census data do not affect population estimates as long as the interstate migration estimates reflect the interstate moves of migrants in their first year in Australia. However, accurate data are not available to measure the extent to which interstate migration estimates cover the movements of migrants within their first year of settlement.
Errors in the estimates of 'category jumping'
4.44. 'Category jumping' arises, in the context of overseas migration statistics, when the duration of a person's journey differs from that originally indicated (on the arrival/departure card at the beginning of the journey) in such a way as to affect his/her categorisation. For example, an Australian resident departing for a short-term visit overseas (with a stated intention to stay abroad for less than twelve months) who in fact stays more than twelve months, thereby changing from short-term to long-term. Changes such as this (ie. between short-term and other categories and vice versa) would lead to errors in post-censal updates of the population. To avoid these errors an adjustment is made to net permanent and long-term migration, when updating population estimates, to compensate for category jumping. However this adjustment cannot be compiled from individual records because of the difficulties attempting to compare arrival and departure records for each individual traveller and because of the complications of lags arising from incomplete journeys. Category jumping must therefore be estimated and is consequently subject to a degree of error. For an outline of the method used in estimating the extent of category jumping see Appendix 3.
Failures to match passenger cards to other data sources
4.46. As detailed in 4.36, data on age, sex and country of birth are obtained through matching to passport or visa records. This matching is done on the basis of the passport number and the citizenship and is done for all fully keyed passenger cards (see Appendix 3). In cases where no match is possible, usually due to an incorrect passport number, no information on age, sex and country of birth is available for that movement and an alternative passenger card of the same citizenship and category of travel is fully keyed. On average, 3,000 to 3,500 alternative passenger cards are selected each month.
Errors from unrecorded arrivals and departures
4.49. These are movements which have not been recorded by any immigration control mechanism and are different from 'category jumping' identified in paragraph 4.44. The numbers are believed to be insignificant although it is not possible to fully quantify them.