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5 The post-censal population estimates are updated each quarter and derived by bringing forward the base population by ageing the base, then adjusting for subsequent components of population growth, i.e. births, deaths, overseas and interstate migration. This method is called the cohort component method.
6 The following equation is known as the demographic balancing equation (Shryock, Siegel and Associates, 1976) and is used to update the base population
7 After each Census, estimates for the preceding intercensal period are revised by incorporating an additional adjustment for residual error (intercensal discrepancy) to ensure that the total intercensal increase agrees with the difference between the estimated resident populations at the two 30 June dates in the respective Census years.
8 A complete explanation of the methods and conceptual basis for population estimates used by the Australian Bureau of Statistics (ABS) in the production of population estimates is given in Population Estimates: Concepts, Sources and Methods, 2009 (cat. no. 3228.0.55.001).
Status of quarterly ERP data
9 Population estimates are referred to as either preliminary, revised or final. Preliminary estimates are generally made available five to six months after the reference quarter. Revised estimates are generally published each March for the financial year ending 21 months previously, and each September for the calendar year ending 21 months previously. Final estimates are published for the previous five-yearly intercensal period after each Census.
10 The following table shows the current status of ERP and the components of population change: natural increase, net overseas migration (NOM) and net interstate migration (NIM).
Population estimates by country of birth
11 Quarterly population estimates by country of birth are compiled and published annually as at 30 June for Australia as a whole. These estimates, produced by single year of age and sex, classify the population according to countries of birth.
12 Quarterly population estimates by country of birth for post-censal years are compiled by updating the Census year estimates in accordance with births, deaths and overseas migration. Each component of change is first converted to financial year of birth. The population for each country of birth by birth cohort are then updated.
13 For more detailed information see Chapter 2 - Estimating National and State Population in Population Estimates: Concepts, Sources and Methods, 2009 (cat. no. 3228.0.55.001).
14 Australia's ERP and estimates of 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
15 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. 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.
16 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
17 The ABS statistics on overseas migration are calculated using administrative data collected and compiled by the Department of Immigration and Citizenship (DIAC) under the authority of the Migration Regulations (Migration Act, 1958). At present, the main source of data on overseas migration is the incoming and outgoing passenger cards completed by all persons arriving in or departing from Australia. Data from passports and visa (entry permit) applications and approvals are also provided by DIAC's Travel and Immigration Processing System (TRIPS). Information from these three data sources are collected, compiled and matched together by DIAC.
18 Quarterly NOM estimates are sourced from the processed monthly overseas arrivals and departures (OAD) data (compiled using matched TRIPS data) and monthly extracts of unmatched TRIPS records. Unmatched TRIPS records are those where a movement has been recorded by DIAC within the TRIPS system but the data has not been able to be matched with either a passenger card, passport or visa permit.
19 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.
20 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
21 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.
22 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.
23 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.
24 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.
25 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.
26 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.
27 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 a '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'
28 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.
29 To measure a traveller's actual duration of stay the ABS uses a unique personal identifier provided with the administrative data supplied by DIAC. 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
30 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.
31 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
32 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
33 Preliminary estimates of NOM are required five to 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.
34 Preliminary estimates using the improved method for estimating NOM 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).
35 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) has enabled two key changes to the methodology for estimating preliminary NOM:
36 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).
37 For further information on the improvements to preliminary NOM estimation and changes to the revision schedule for NOM, see the Information Paper: Improving Net Overseas Migration Estimation, Mar 2010 (cat. no. 3412.0.55.001). For further information on the '12/16 month rule' methodology see the Technical Note: '12/16 month rule' Methodology for Calculating Net Overseas Migration from September quarter 2006 onwards in this publication. For more detailed information see Information Paper: Statistical Implications of Improved Methods for Estimating Net Overseas Migration, Australia, 2007 (cat. no. 3107.0.55.005), and the Information Paper: Improved Methods for Estimating Net Overseas Migration, Australia, 2006 (cat. no. 3107.0.55.003).
Estimating NOM with a '12/12 month rule'
38 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).
39 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.
40 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.
41 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 DIMA (now DIAC) to derive actual duration of stay or absence by matching both arrival and departure cards rather than relying on passengers reporting their duration of stay or absence.
Matching traveller movements
42 Despite this improvement in the quality of actual duration of stay or absence data, 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.
43 Through the provision of additional data from DIAC, 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.
44 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
45 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
46 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. The proportions of temporary arrivals subsequently gaining onshore grants of permanent residency are not estimated in ABS statistics.
47 For more information on onshore additions to the population see the DIAC publication Immigration Update, or Population Flow: Immigration aspects available on the DIAC web site, <http://www.immi.gov.au>.
NET INTERSTATE MIGRATION
48 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.
49 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
50 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.
51 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.
52 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
53 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.
54 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
55 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.
56 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
57 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.
58 For further information on the process of estimating interstate migration and the administrative data used, see the Demography Working Paper: 2004/1 Review of Interstate Migration Method (cat. no. 3106.0.55.001) and the Information Paper: Evaluation of Administrative Data Sources for Use in Quarterly Estimation of Interstate Migration, 2006 to 2011 (cat. no. 3127.0.55.001) and Population Estimates: Concepts, Sources and Methods, 2009 (cat. no. 3228.0.55.001).
59 The classification of countries in this publication is the Standard Australian Classification of Countries. For more detailed information, refer to the ABS publication Standard Australian Classification of Countries (SACC) Second Edition (cat. no. 1269.0). This replaced previous revision and the Australian Standard Classification of Countries for Social Statistics (ASCCSS) used in earlier issues of this publication.
60 The statistics on country of birth, citizenship, residence or main destination 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. Similarly the United States of America includes 'America (undefined)'.
STATE AND TERRITORY CLASSIFICATION
61 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'.
62 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.
ADDITIONAL STATISTICS AVAILABLE
63 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>.
64 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.
65 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:
66 This publication draws extensively on information provided by DIAC. 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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