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10 Australia's ERP and estimates of net overseas migration (NOM) include all people, regardless of nationality or citizenship, who usually live in Australia, with the exception of foreign diplomatic personnel and their families. Therefore, foreign diplomatic personnel and their families are considered out of scope and were removed from NOM estimates from 1 July 2006. The previous methodology for estimating NOM was unable to exclude diplomatic personnel and their families. However, with the improved NOM methodology, refinements to the NOM processing system have enabled this to occur through the use of visa information.
NET OVERSEAS MIGRATION
11 According to recommendations of the United Nations an international migrant is defined as "any person who changes his or her country of usual residence" (United Nations 1998). For the purposes of estimating NOM, and thereby Australia's official ERP counts, a person is regarded as a usual resident if they have been (or expected to be) residing in Australia for a period of 12 months or more over a 16 month period. As such, NOM and ERP estimates include all people, regardless of nationality, citizenship or legal status, who usually live in Australia, with the exception of foreign diplomatic personnel and their families.
12 Conceptually, the term NOM is based on an international travellers' duration of stay being in or out of Australia for 12 months or more. It is the difference between the number of incoming travellers who stay in Australia for 12 months or more and are added to the population (NOM arrivals) and the number of outgoing travellers who leave Australia for 12 months or more and are subtracted from the population (NOM departures). With the introduction of the improved methods for estimating NOM, this 12 months does not have to be continuous and is measured over a 16 month reference period. For example whether a traveller is in or out of the population is determined by their exact duration of stay in or away from Australia over the subsequent 16 months after arrival or departure.
Source of overseas migration data
13 The ABS statistics on overseas migration are calculated using administrative data collected and compiled by the Department of Immigration and Border Protection (DIBP) under the authority of the Migration Regulations (Migration Act, 1958). The main source of data on overseas movements is incoming and outgoing passenger cards, matched with data from passports and visa permits. Information from these three data sources are collected, compiled and matched together by DIBP and stored with movement records on their Travel and Immigration Processing System (TRIPS). Each month these matched overseas arrivals and departures (OAD) records are supplied to the ABS and then processed within the OAD system.
14 Quarterly NOM estimates are sourced from this processed monthly OAD matched data and then combined with monthly extracts of unmatched OAD records. Unmatched OAD records are those where an inward/outward movement has been recorded by DIBP within the TRIPS system, but the data has not been able to be matched with an equivalent passenger card.
15 Statistics on overseas migration exclude: multiple movements; the movements of operational air and ships' crew; transit passengers who pass through Australia but are not cleared for entry; passengers on pleasure cruises commencing and finishing in Australia and undocumented arrivals. From 1 July 2006 onwards, foreign diplomatic personnel and their families are also excluded.
16 Quarterly NOM estimates contribute to quarterly ERP and are released in Australian Demographic Statistics (cat. no. 3101.0). Statistics on OAD and related data quality issues are published on a monthly basis in Overseas Arrivals and Departures, Australia (cat. no. 3401.0).
Reasons for change of methods used to estimate NOM
17 During a reconciliation of 1996-2001 intercensal population growth estimates (including measures of immigration, emigration and NOM) with the results of the 2001 Census of Population and Housing, the ABS identified that inconsistent results were arising from a 1998 passenger card processing change and the measurement of temporary visitors' duration of stay in Australia, or Australian residents' temporary duration of absence from Australia.
18 The ABS noted that the precise measurement of duration of stay in Australia or absence from Australia using a comparison of border crossing transactions may lead to a misleading categorisation of frequent travellers to short-term, when their overall period of stay or absence in a broader context was long-term punctuated by short journeys. For example, an international student in Australia for a three or four year course of study, who leaves Australia briefly each year for holiday or other reasons, would incorrectly not be added to estimates of Australia's population. This was because after the passage of time, they did not spend a continuous period of 12 months in Australia during their long-term period of stay in Australia (the previous method of measurement). This was inconsistent with the intention behind the definition of a long-term migrant as set out in the United Nations Recommendations on Statistics of International Migration, Revision 1 (1998, 18) and the ABS Estimated Resident Population conceptual definition. The use of single continuous periods of stay as the criteria (i.e. 12 continuous months) for classifying travellers into the non-permanent migration categories (e.g. short-term visitor, long-term visitor) was seen as a major shortcoming in the measurement of NOM and consequently estimates of Australia's resident population.
19 Further, there were some travellers such as permanent immigrants and emigrants who were not asked their intended duration of stay or absence on Australian passenger cards, yet it was observed that after a passage of less than a year, some permanent immigrants departed Australia and some permanent emigrants returned to Australia. The ABS also identified a need to convert the multiple border movements information for frequent travellers within a reference period to individual person estimates together with their travel history over time to avoid double counting.
20 The method for estimating NOM was reviewed in 2004 in response to the issues arising with the previous estimation of category jumping, i.e. changes between stated intention and actual duration of stay of travellers to/from Australia. The review also addressed the changing patterns of travel into and out of Australia, in particular the increased propensity for travellers to interrupt longer periods of stay or absence with short-term trips.
21 The methodology and estimation system adopted by the ABS for measuring NOM and the contribution to Australia's ERP more closely aligns with the international definitions set out by the United Nations. The ABS has not changed the statistical conceptual definition of net overseas migration. However, the method of measurement has changed.
22 The ABS developed and introduced an improved method, called the '12/16 month rule' methodology, for estimating NOM. It has been used in calculating Australia's official ERP since September quarter 2006. The '12/16 month rule' methodology is a result of reviewing the treatment of temporary migrants (both long-term and short-term) who are away from or resident in Australia for a period of 12 months or more.
23 Estimates of NOM based on the previous methods and those based on the '12/16 month rule' methodology are not comparable. The key change is the introduction of the '12/16 month rule' for measuring a person's residency in Australia, replacing the previous '12/12 month rule'.
Estimating NOM with the '12/16 month rule'
24 The current NOM estimation methods employ a '12/16 month rule' where the traveller can be added or subtracted from NOM if they have stayed in or been absent from Australia for a period of 12 months or more over a 16 month period. This period of 12 months does not have to be continuous. Although a traveller states their intended duration of stay on a passenger card, for NOM purposes the ABS now measures an individual's actual travel behaviour (or models this behaviour for preliminary NOM).
25 To measure a traveller's actual duration of stay the ABS uses a unique personal identifier provided with the administrative data supplied by DIBP. To be able to apply the '12/16 month rule', the personal identifier is used to match a traveller's movements over time and construct a movement history for each arrival and departure record.
Travellers vs movements
26 Conceptually, NOM estimates should be based on counts of travellers, rather than counts of overseas movements, since travellers may have more than one movement in a particular reference period. Under the previous system of NOM estimation, a number of adjustments to overseas arrivals and departures were required. These mainly comprised adjustments designed to reflect differences between stated travel intentions and actual travel behaviour. However, adjustments were also required to transform numbers of overseas movements into numbers of travellers.
27 One of the central changes with the '12/16 month rule' methodology is that all estimation is based on actual individual travellers and their travel histories (using de-identified data), rather than in the previous methodology when an aggregation of movements represented travellers.
Final NOM estimates
28 It is with the final NOM estimates that the '12/16 month rule' can be fully applied. A traveller's actual duration of stay can only be calculated when data on overseas movements become available for the 16 months following a reference period. Final NOM estimation methods use ERP flags to determine if a traveller, through their actual duration of stay in or out of Australia, should be included or excluded from NOM estimates and consequently ERP estimates.
Preliminary NOM estimates
29 Preliminary estimates of NOM are required within six months after the reference quarter for the production of quarterly estimates of the population of Australia, and the states and territories. At that time, complete traveller histories for the 16 months following a reference quarter cannot be produced. Migration adjustments are calculated from changes in behaviour from final estimates one year earlier for the same groups of travellers. These migration adjustments are applied to travellers who are grouped according to their 'initial category of travel', age, country of citizenship and state/territory of usual/intended residence. The adjustments account for differences between their intended duration of stay and their actual duration of stay.
30 Preliminary estimates using the '12/16 month rule' method for estimating NOM originally using a 'two year ago' propensity model were implemented in official ABS population estimates for September quarter 2006 with the release of the December quarter 2006 issue of Australian Demographic Statistics (cat. no. 3101.0).
31 In 2009, changes to the Federal Financial Relations Act 2009, enabled the use of an additional quarter of travellers' movement data allowing for the methodology used in preliminary NOM estimates to be improved. Using the additional one quarter of movement data (the quarter after the reference period) enabled two key changes to the methodology for estimating preliminary NOM:
32 Preliminary estimates using the '12/16 month rule' methodology for estimating NOM using the 'one year ago' propensity model were implemented in the ABS' official NOM and population estimates for September quarter 2008 with the release of the September quarter 2009 issue of Australian Demographic Statistics (cat. no. 3101.0).
33 For further information on the improvements to preliminary NOM estimation and changes to the revision schedule for NOM, see:
34 For further information on the '12/16 month rule' methodology see:
Estimating NOM with a '12/12 month rule'
35 Prior to 1 July 2006, NOM estimation methods used a '12/12 month rule' to determine if a traveller contributed to ERP. This meant that in order for a person to contribute to NOM they must stay in or be absent from Australia for a continuous period of 12 out of 12 months. It compared data on actual travel movements over a 12 month period with data on individual travellers' duration of stay as recorded on their passenger cards. In order to conduct such a comparison, data for a 15 month period (i.e. one year plus one quarter) were required. For more information see the Technical Note in Migration, Australia, 2006-07 (cat. no. 3412.0) - Measuring Net Overseas Migration, Method Used September quarter 2001 to June quarter 2006 and Demography Working Paper 2003/5 - Net Overseas Migration: Adjusting for Actual Duration of Stay or Absence (cat. no. 3137.0).
36 Many overseas travellers stay (or are away) shorter or longer periods than initially intended, as recorded on their passenger cards. From July 1982 to June 1997, NOM estimates included an adjustment for the net effect of category jumping. Category jumping is a measure of the discrepancy between movements recorded as short-term, long-term or permanent at the time of movement, and the category recorded at the completion of a journey. Twelve months after a reference period it was determined whether the number of initially-recorded short-term, long-term and permanent arrivals and departures matched actual patterns of movement.
37 For example, some visitors on arrival may state that they intend to stay in Australia for more than 12 months. However, they may change their travel plans and depart the country after only six months. Since migration figures were affected by this change in travel behaviour, an adjustment was incorporated into the NOM estimate and ERP.
38 The method used to estimate category jumping up until June 1997 inclusive was based on aggregate flows of traveller movements rather than individual travellers. Until June 1998, the measurement of duration of stay or absence on the second leg of travel was based on passenger reporting on the arrival or departure card. This self reported duration was used to determine the time at which a person arrived (for visitors) or left Australia (for Australian residents). However, from July 1998 onwards, implementation of a new passenger card design and processing system enabled the DIMA (now DIBP) to derive actual duration of stay or absence for the second leg of travel by matching both the actual arrival and departure movement dates rather than relying on passengers reporting their duration of stay or absence.
Matching traveller movements
39 Despite this improvement in the quality of actual duration of stay or absence data for the second leg of travel, the above estimation method appeared incapable of producing acceptable estimates of category jumping. Given that category jumping had only a small effect on ERP and that estimates produced by the above method seemed highly volatile, the ABS decided to set category jumping estimates to zero until an improved estimation technique was developed. They were set to zero from September quarter 1997 to June quarter 2001.
40 Through the provision of additional data from DIBP, the ABS had the ability to match traveller movements over time. This enabled a movement history to be constructed for those arriving and departing and thus calculate an actual duration of stay. Matching traveller movements enabled the adjustment of permanent and long-term movement. This adjustment (termed 'migration adjustment') allowed for components of NOM to be presented on an adjusted basis.
41 For more information on category jumping and the interim methods of adjusting NOM for the previous (12/12) method, see Demography Working Paper 2003/5 - Net Overseas Migration: Adjusting for Actual Duration of Stay or Absence (cat. no. 3137.0). Adjustments applied to overseas migration estimates have also been discussed in a special article in Migration, Australia, 2002-03 (cat. no. 3412.0).
ADJUSTMENT METHODS AND REVISION STATUS
42 Due to changes in the methods used to adjust NOM estimates, caution should be used comparing estimates over time. The table below describes the adjustment methods that have been applied to NOM estimates from September quarter 1996 and onwards.
PERMANENT RESIDENCY GRANTS
43 A number of people arriving temporarily in Australia are subsequently granted permanent residency. These permanent residency grants contribute to the Australian Government's immigration targets but may be unrelated to the stated intentions of travellers on arrival. Accordingly, they are not included in unadjusted permanent arrivals as they did not arrive in Australia on a permanent basis but would be included in final NOM figures from 1 July 2006 onwards (if satisfying the "12/16 month rule"). The proportions of temporary arrivals subsequently gaining onshore grants of permanent residency are not estimated in ABS statistics.
44 For more information on onshore additions to the population see the DIBP publication Immigration Update, or Population Flow: Immigration aspects available on the DIBP web site, <http://www.immi.gov.au>.
OVERSEAS ARRIVALS AND DEPARTURES (OAD) STATISTICS
45 The ABS statistics on overseas arrivals and departures (OAD) are calculated using administrative data collected and compiled by the Department of Immigration and Border Protection (DIBP) under the authority of the Migration Regulations (Migration Act, 1958). The main source of data on overseas movements is incoming and outgoing passenger cards, matched with data from passports and visa permits. Information from these three data sources are collected, compiled and matched together by DIBP and stored with movement records on their Travel and Immigration Processing System (TRIPS). Each month these matched overseas arrivals and departures (OAD) records are supplied to the ABS and then processed within the OAD system.
46 In July 1998, DIBP revised the incoming and outgoing passenger cards and associated procedures as well as computer systems. Following these changes, some questions on the passenger cards were not compulsory and answers to these questions were not checked by Customs officers. The question on marital status was deleted. Data on marital status are now derived from visa applications (only for certain visa classes) and are therefore not available for Australian or NZ citizens. The changes also affect the data for 'previous country of residence' which is imputed for Australian and NZ citizens. For more information see the May 1998 issue of Overseas Arrivals and Departures, Australia (cat. no. 3401.0). Since July 1998, there have been additional minor changes to both incoming and outgoing passenger cards.
47 From July 2001, DIBP adopted a new passenger card processing system which involved electronic imaging of passenger cards and intelligent character recognition of the data stored in the images. This process has yielded several improvements to the processing of passenger card data, most notably the detailed information about missing values. There have also been several changes to data quality. Further information on these changes is provided in Overseas Arrivals and Departures, Australia (cat. no. 3401.0).
Rebuild of the OAD system
48 In 2013, the ABS completed a rebuild of the OAD system (ROADS). The primary aim of this project was to improve the quality of OAD data, given its importance as input to a broad range of statistical collections, including the Net Overseas Migration (NOM) collection and the Estimated Resident Population (ERP) by country of birth collection. It improved the imputations for missing traveller data, in particular the duration of stay and the country of birth variables. The rebuild of the system has produced a revised time series for OAD data from July 2004 to December 2013. Detailed information on the changes and improvements made with the complete rebuild of the OAD system (ROADS), and the new OAD data time series from July 2004, will be made available with the release of Overseas Arrivals and Departures, Australia, January 2014 (cat. no. 3401.0) scheduled for release on 11 March 2014.
49 Overseas arrivals and departures statistics relate to the number of movements of travellers rather than the number of travellers (i.e. multiple movements of individual persons during a given reference period are each counted separately). The statistics exclude the movements of operational air and ships' crew, of transit passengers who pass through Australia but are not cleared for entry, and of passengers on pleasure cruises commencing and finishing in Australia. Similarly, these statistics exclude unauthorised arrivals.
NET INTERSTATE MIGRATION
50 At the national level, population change is the result of births, deaths and net overseas migration. At the state/territory level, an extra component of population change exists - net interstate migration (NIM). This is the net difference between arrivals to a state/territory from the rest of Australia and departures from that state/territory to the rest of Australia. Interstate migration is therefore an important determinant of population change and distribution of the states and territories.
51 Within Australia, there is no requirement for a person who changes their state of usual residence to register their move. Unlike overseas movements, which are recorded at Australia's borders, there are no direct quarterly measure of arrivals and departures between the states and territories. To be able to measure state/territory population change on a quarterly basis estimates of interstate migration are therefore required.
Sources of interstate migration data
52 The Census is one source of information, with people being asked where they lived one year ago and five years ago. However, as the Census is held only every five years, this is insufficient for producing quarterly interstate migration estimates. Another source of data is therefore necessary.
53 Interstate migration is a key determinant of the accuracy of state and territory population estimates. Data on interstate migration cannot be directly estimated unlike that of natural increase and net overseas migration. Instead, post-censal quarterly estimates of interstate migration are modelled using administrative by-product data. Over time, the ABS has used a number of administrative data sources to produce quarterly estimates of interstate migration, including electoral roll registrations and family allowance payments. Currently the ABS uses information on interstate change of address advised to Medicare Australia and to the Department of Defence in the case of the military.
54 Due to incomplete coverage and the non-compulsory nature of available administrative (indirect) data sources, post-censal quarterly estimates of interstate migration have long been considered the weakest measure of a component of population change.
Rebasing and re-derivation of interstate migration
55 The Medicare-based model used for generating post-censal estimates of interstate migration is largely superseded when new Census information becomes available. For example, every five years, after data from the following Census have been finalised, the modelled estimates are reviewed against, and potentially replaced by, the interstate migration estimates that are calculated from the Census (i.e. rebased to the Census). This is known as the re-derivation of interstate migration.
56 Part of the process of rebasing Census counts for the ERP of the states and territories is the re-derivation of interstate migration for the intercensal period. The overall approach is to minimise state intercensal discrepancy using information from the two Census questions on usual residence one year ago and five year ago to estimate interstate movements. Where this Census information does not reduce the intercensal discrepancy, the rebased interstate migration estimates remain largely unchanged from the Medicare-based model.
Interstate migration method
57 Post-censal quarterly estimates of net interstate migration are created for the states and territories (excluding Other Territories) using interstate change of address advised to Medicare Australia and to the Department of Defence in the case of the military. Medicare data are adjusted by means of expansion factors. These expansion factors are used to account for an under coverage of Medicare data by various ages and sex. For example, it is known that some people, particularly younger Medicare card holders, do not register changes of address with Medicare, or do so long after the fact.
58 Expansion factors are used in the calculation of post-censal quarterly estimates of net interstate migration and remain constant throughout the intercensal period until once again they are reviewed after final data from the following Census of Population and Housing becomes available. They are calculated for each state and territory (excluding Other Territories), single year of age, sex and movement direction (i.e. arrivals or departures).
Defence force adjustment
59 Adjustments to compensate for interstate defence force movements not covered by Medicare are applied to the quarterly interstate migration estimates. These adjustments are estimated using counts of defence force personnel by age, sex and state/territory, obtained from the Department of Defence, with 70% of any change in quarterly defence force numbers assumed to be due to interstate migration not otherwise covered by the Medicare model.
60 Due to the fact that the Medicare and Defence data source is an indirect measure of interstate migration, the post-censal quarterly estimates of interstate migration have long been considered the weakest measure of a component of population change at the state and territory level. For further information on the process of estimating interstate migration and the administrative data used, see
61 The classification of countries in this publication is the Standard Australian Classification of Countries, 2011, Version 2.2. For more detailed information, refer to the ABS publication Standard Australian Classification of Countries (SACC), 2011 (cat. no. 1269.0) or the Country Classification excel spreadsheet in the Downloads tab of this publication. This has replaced the time series available and all previous versions and previous classifications.
62 The statistics on country of residence or main destination, and country of embarkation or disembarkation have certain limitations because of reporting on passenger cards. For instance, the United Kingdom, Channel Islands and Isle of Man (UK, CI & IOM) includes England, Scotland, Wales, Northern Ireland, Guernsey, Jersey and the Isle of Man. Many travellers just list the UK. Similarly the United States of America includes 'America (undefined)'.
STATE AND TERRITORY CLASSIFICATION
63 Prior to the 1996 Census, no external territories were included in geographical Australia although Census data were collected for Christmas Island and the Cocos (Keeling) Islands. Following amendments to the Acts Interpretation Act 1901 effective from July 1992, the two external territories of Christmas Island and Cocos (Keeling) Islands became part of geographical Australia. Since the 1996 Census, Christmas Island, Cocos (Keeling) Islands, and the Jervis Bay Territory (previously linked to the Australian Capital Territory for statistical purposes) comprise a pseudo 'ninth state/territory' of Australia. They are included in state nine 'Other Territories'.
64 Although the Census and Statistics Act 1905 does not require quarterly estimation of the population for the territories, estimates for the Northern Territory, the Australian Capital Territory and the Other Territories are produced as these territories are included in the geographical area of Australia, and, with the states, sum to the Australian population.
65 The Census and Statistics Act, 1905 provides the authority for the ABS to collect statistical information, and requires that statistical output shall not be published or disseminated in a manner that is likely to enable the identification of a particular person or organisation. This requirement means that the ABS must take care and make assurances that any statistical information about individual respondents cannot be derived from published data.
66 Some techniques used to guard against identification or disclosure of confidential information in statistical tables are suppression of sensitive cells, random adjustments to cells with very small values and rounding. In these cases data may not sum to totals due to the confidentialisation of individual cells.
67 To minimise the risk of identifying individuals in aggregate statistics, a technique is used to randomly adjust cell values. This technique is called perturbation. Perturbation involves small random adjustment of the statistics and is considered the most satisfactory technique for avoiding the release of identifiable statistics while maximising the range of information that can be released. These adjustments have a negligible impact on the underlying pattern of the statistics. After perturbation, a given published cell value will be consistent across all tables. However, adding up cell values to derive a total will not necessarily give the same result as published totals. The introduction of perturbation in publications ensures that these statistics are consistent with statistics released via services such as Table Builder.
68 In this publication, estimates have sometimes been rounded. Rounded figures and unrounded figures should not be assumed to be accurate to the last digit shown. Where figures have been rounded, discrepancies may occur between sums of component items and totals.
ADDITIONAL STATISTICS AVAILABLE
69 Additional demographic information is available on the ABS web site, Themes - Demography page. Users can also access the full range of electronic ABS data free of charge on the ABS web site <https://www.abs.gov.au>.
70 The ABS may have other relevant data available on request. Generally, a charge is made for providing this information. Inquiries should be made to the National Information and Referral Service on 1300 135 070.
71 With the introduction of '12/16 month rule' methodology for estimating NOM, the ABS also developed an analytical data set called the Travellers' Characteristics Database. The improvements allow the derivation of an individual's actual true travel behaviour (using final NOM data) and record certain characteristics for any traveller who has contributed to NOM whether they are a NOM arrival or a NOM departure. The database provides for additional analysis on final NOM data that was not previously available. The following variables may be made available on request for final data only:
72 This publication draws extensively on information provided by DIBP, Medicare and the Department of Defence. The ABS also uses information provided freely by individuals, businesses, governments and other organisations. Their continued cooperation is very much appreciated; without it, the wide range of statistics published by the ABS would not be available. Information received by the ABS is treated in strict confidence as required by the Census and Statistics Act 1905.
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