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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 (e.g. revised estimates for the 2006-07 financial year were available in March 2009). 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 Prior to the introduction of the improved NOM method from July 1 2006 there had been concern about the reliability of long-term overseas migration data, including the manifestation of some countries with many long-term arrivals but far fewer departures. Therefore between 2001 and 2006 overseas migration by country of birth was derived from permanent flows only, constrained to net Australian levels.
14 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).
15 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
16 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 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 or citizenship, who usually live in Australia, with the exception of foreign diplomatic personnel and their families.
17 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
18 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). These three data sources are collected, compiled and matched together by DIAC.
19 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.
20 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 unauthorised arrivals. From 1 July 2006 onwards foreign diplomatic personnel and their families are also excluded.
21 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).
Change of methods used
22 The ABS has developed and introduced an improved method for estimating NOM. It has been used in calculating Australia's official ERP since September quarter 2006. The improved method 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 improved methods 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 12/16 rule
24 The method for estimating NOM was reviewed in 2004 in response to 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 improved NOM estimation methods employ a 12/16 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 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 individuals' actual travel behaviour.
26 To measure a travellers 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 rule the personal identifier is used to match a travellers movements over time and construct a movement history for each arrival and departure record.
Travellers vs movements
27 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.
28 One of the central changes with the improved 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
29 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 becomes 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
30 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, 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 two years earlier for the same groups of travellers. These migration adjustments are applied to travellers who are grouped according to age, sex, country of citizenship and state/territory, and account for differences between their intended duration of stay and their actual duration of stay.
31 Preliminary estimates using the improved method for estimating NOM were implemented in official ABS population estimates for September quarter 2006 and onwards with the release of the December quarter 2006 issue of Australian Demographic Statistics (cat. no. 3101.0).
32 For further information on the improved methods see the Technical Note: Improved Methods of Calculating Net Overseas Migration from September quarter 2006 onwards in this publication. For more detailed information on the improved NOM estimation methods see Information Paper: Statistical Implications of Improved Methods for Estimating Net Overseas Migration, Australia, 2007 (cat. no. 3107.0.55.005) and Information Paper: Improved Methods for Estimating Net Overseas Migration, Australia, 2006 (cat. no. 3107.0.55.003).
ESTIMATING NOM WITH THE 12/12 RULE
33 Prior to 1 July 2006, NOM estimation methods used a 12/12 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).
34 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.
35 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.
36 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
37 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.
38 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.
39 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
40 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
41 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 short-term and long-term visitor arrivals subsequently gaining on-shore grants of permanent residency are not routinely estimated in ABS statistics.
42 For more information on permanent additions to the population see the DIAC publication Immigration Update, available on the DIAC web site, <http://www.immi.gov.au>.
NET INTERSTATE MIGRATION
43 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 growth and distribution of the states and territories.
44 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
45 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.
46 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 data used by the ABS is information on interstate change of address advised to Medicare Australia and to the Department of Defence in the case of the military.
47 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 has 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.
Rebasing and re-derivation of interstate migration
48 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. For this reason, the model for generating post-censal estimates of interstate migration is largely superseded when new Census information becomes available (i.e. rebased to the Census).
49 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
50 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 card holders, do not register changes of address with Medicare, or do so long after the fact.
51 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
52 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.
53 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).
CENSUS COUNTS - POPULATION MOBILITY
54 Censuses provide comprehensive data on the Australian population, cross-classified by a wide range of socio-economic characteristics and for a variety of geographic areas. It is also the optimal source for small area data and sub-populations. These data are referred to as 'Census counts' and are available as at Census dates only.
55 Population mobility in this publication refers to the geographic movement of people where there has been a change in the place of usual residence. It is measured using census data based on the place of usual residence of each individual at the time of the latest census and is compared with data based on the place of usual residence one year earlier and five years earlier as asked on the census form.
56 The Census asks a series of questions relating to each person's usual address on Census night, one year ago and five years ago. The indicative data from these questions are recorded as the Usual Address Indicator Census Night (UAICP), Usual Address One Year Ago Indicator (UAI1P) and Usual Address Five Years Ago Indicator (UAI5P). The combination of various Census data allows for the measurement and analyses of mobility patterns over time.
57 The geographic movement of individuals derived from data collected in the census only reflects the latest movement in the intercensal period, even though there may have been multiple movements during this period for some individuals.
58 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.
59 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
60 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'.
61 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
62 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>.
63 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.
64 With the introduction of improved methods for estimating NOM and a new NOM processing system the ABS has developed an analytical data set called the Travellers Characteristics Data Base (based on data from final NOM processing using the 12/16 month rule). The following variables may be made available on request for final data only:
65 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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