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1 Estimates of the Australian population are generated on a quarterly basis by adding natural increase (the excess of births over deaths) and net overseas migration (NOM) occurring during the period to the population at the beginning of each period. This is known as the cohort component method, and can be represented by the following equation:
P(t+1) = P(t) + B - D + NOM, where:
2 For state and territory population estimates, an additional term is added to the equation representing net interstate migration occurring between t and t+1.
3 Net overseas migration accounts for around half of population growth at the national level. This note outlines how the ABS calculates NOM estimates by state and territory, including adjustments made to overcome some limitations of existing migration data.
4 The ABS estimates the level of NOM occurring during each quarter using data on incoming (i.e. arriving) and outgoing (i.e. departing) passenger movements at Australian air and sea ports. These movements are classified into three main categories depending on the stated duration of stay in Australia or overseas:
5 Conceptually, NOM is the difference between permanent and long-term arrivals, and permanent and long-term departures. However, at the time a person crosses the Australian border, it is not empirically known how long they will actually spend in Australia or overseas. For example, overseas visitors might change their travel plans and extend their stay in Australia (perhaps utilising on-shore visa grants), or depart earlier than they first intended. Similarly, Australian residents travelling overseas may change their plans while abroad (e.g. some might state that they are departing the country permanently, but return less than a year later, while others might stay overseas longer than they initially intended).
6 Some of these differences between stated travel intentions and actual travel behaviour may also reflect short interruptions to longer periods of stay or absence. For example, overseas students arriving in Australia might state that they intend to stay for three years, but return home for brief periods during this time. Similarly, Australians working or studying overseas might state that they intend to be away for more than a year but return for brief holidays.
7 The following diagram summarises the contributions of different types of overseas movements to NOM. Estimates of NOM are derived from information provided on incoming and outgoing passenger cards, as well as other data supplied by the DIMIA. Data on the intended duration of stay of overseas visitors arriving in Australia and the intended duration of absence of Australian residents travelling overseas are used to determine the numbers of permanent and long-term arrivals, and permanent and long-term departures. Passenger card data are also used to calculate migration adjustments and determine the state and territory distribution of NOM.
9 The processes of adjusting movement data on travellers' stated intentions to reflect their actual behaviour are complex, and depend upon the amount and type of movement data available at a particular point in time. The methods currently used compare data on actual travel movements over a one year period with those first advised by individual travellers, and are explained in more detail in Demography Working Paper 2003/5 - Net Overseas Migration: Adjusting for Actual Duration of Stay or Absence (<http://www.abs.gov.au>, select Themes> Demography > ABS Demography Working Papers). In order to conduct such a comparison, data for a 15month period (i.e. one year plus one quarter) are required. These adjustment methods described in the working paper have been applied to NOM data from the September quarter 2001 onwards and will be subject to further investigation and improvement with the accumulation of additional data and time series.
10 Table 1 below describes the impact that various types of migration adjustments have on NOM estimates. The adjustments applied to preliminary and revised NOM estimates are described in more detail elsewhere in this document.
TABLE 1 - MIGRATION ADJUSTMENTS APPLIED TO NOM ESTIMATES
(b) Based on matched passenger records comparing stated travel intentions with actual behaviour.
(c) Numbers of movements are converted into numbers of persons by matching passport numbers and other identifying personal details.
State and territory distribution of NOM
11 The state or territory distribution of NOM is based on information reported by travellers on arrival in or on departure from Australia. Incoming passenger cards provide information on the state or territory of a traveller's intended address within Australia, while outgoing passenger cards provide information on the state or territory in which a traveller lives or spent most time. However, the way in which this distribution is calculated differs between preliminary and revised estimates of NOM due to the amount of data available.
12 The following sections of this document describe how preliminary and revised estimates of NOM are created and distributed between states and territories. Estimates of NOM are finalised after the five-yearly Census of Population and Housing.
PRELIMINARY NOM ESTIMATES
13 The ABS produces quarterly estimates of Australia's resident population (known as the ERP) five to six months after the end of the reference quarter, and is required under legislation to provide population estimates as at 31 December by early June of the following year. Since estimates of NOM (adjusted for actual travel behaviour) require 15 months of data, preliminary estimates of NOM are calculated to meet more immediate ERP requirements.
14 There are four main groups of travellers who provide an intended duration of stay on their passenger cards who have the potential to change their duration of stay or absence:
15 Migration adjustments applied to preliminary NOM estimates are based on the trends observed for the proportions of long-term and short-term arrivals and departures who change their travel behaviour. Table 2 shows the proportion of long-term and short-term travellers in 2001-02 who had changed their stated travel intentions. Preliminary migration adjustments are only applied to the four major movement categories (i.e. long-term visitor arrivals, short-term visitor arrivals, long-term visitor departures and short-term resident departures).
TABLE 2 - CHANGES IN TRAVEL BEHAVIOUR(a), Selected categories of movement(b) - September quarter 2001 to June quarter 2002
(b) Based on stated intentions.
16 An average adjustment based on the most recent complete financial year for which 15 months of data exist is applied to each new quarter of movement data. For example, preliminary NOM estimates for the June quarter 2003 and September quarter 2003 each assumed that, based on the 2001-02 evidence, 70.0% of long-term visitor arrivals during the quarter would in fact stay in Australia for less than 12 months, while 49.5% of long-term resident departures would return to Australia within 12 months. These preliminary data are expected to be revised in the March 2004 and March 2005 issues, respectively, of Australian Demographic Statistics (cat. no. 3101.0).
17 Table 3 below shows how the preliminary NOM estimate for the September quarter 2003 was calculated.
3. COMPONENTS OF NET OVERSEAS MIGRATION, Original and adjusted estimates - September quarter 2003
(a) Refer to Table 1 in this document for further information on the migration adjustments applied to preliminary NOM estimates.
State and territory distribution
18 As noted above at paragraph 10, the state or territory distribution of NOM is based on information reported by travellers on arrival in or on departure from Australia. However, at the time preliminary NOM estimates are calculated, information on the state or territory in which long-time arrivals will actually spend most time in is not available because outgoing passenger cards for these persons have not yet been completed. State and territory distributions of long-term arrivals therefore refer to the state or territory of their intended addresses, as advised on incoming passenger cards. Similarly, state and territory distributions of permanent arrivals refer to their intended addresses as advised on incoming passenger cards, which may differ from the state or territory where they settle in the longer term.
19 The state and territory distribution of preliminary migration adjustments for a particular quarter is assumed to be the same as that of permanent and long-term arrivals in the same quarter. In practice, a national total is calculated for the migration adjustment. This is then distributed across the states and territories, by age and sex, using the distribution of permanent and long-term arrivals by state or territory of intended address. For example, since 24.8% of all permanent and long-term arrivals in the September quarter 2003 intended to live in Victoria, 24.8% of the total migration adjustment (-3,783) is also applied to this state. Table 4 shows components of net overseas migration for September quarter 2003 by state and territory.
4. COMPONENTS OF NET OVERSEAS MIGRATION, States and territories - September quarter 2003
21 However, the ABS plans to review this method, with the prospect of applying a distribution method which allows for positive as well as negative adjustments for individual states and territories. In the interim, the preliminary estimates of NOM are subject to revision when more complete data are available.
REVISED NOM ESTIMATES
22 Preliminary estimates of NOM for a financial year are usually revised in the following March issue of Australian Demographic Statistics (cat. no. 3101.0). These revised NOM estimates use matched passenger records to calculate the actual duration of stay relating to overseas movements. Migration adjustments applied to NOM estimates are based on these matched data and include, in addition to the four major movement categories previously identified, a subset of movements relating to permanent arrivals and permanent departures:
23 Migration adjustments applied to revised NOM estimates also adjust for multiple movements of travellers (i.e. converting numbers of movements into numbers of persons).
24 The current methodology for these revised migration adjustments has been applied from the September quarter 2001 to June quarter 2002. Table 5 shows how revised NOM estimates were calculated for 2001-02.
5. COMPONENTS OF NET OVERSEAS MIGRATION, Original and adjusted estimates - 2001-02
25 As is the case for preliminary NOM estimates, the state and territory distribution of revised NOM estimates is determined based on information reported on incoming and outgoing passenger cards (i.e. state or territory of intended address for arrivals and state or territory of residence/spent most time for departures).
26 The state and territory distributions of the migration adjustment are calculated based on the initial passenger card that identifies the movement of the traveller. For example, a long-term resident departure who returned to Australia within twelve months is added back to the state of residence they reported on departure (as identified on their outgoing passenger card). A long-term visitor arrival who actually stayed in Australia for less than twelve months is taken away from the state or territory they intended to live in (as identified on their incoming passenger card).
27 This method may be considered to be reasonable for people who, on arrival, intend to settle or stay in Australia for more than twelve months. However, there is less certainty about the reliability of the state or territory of intended stay for those persons who originally stated that they intended to stay for less than twelve months, but actually stayed longer, and this component of the migration adjustment is treated differently.
28 In the absence of direct information from outgoing passenger cards for this group, the ABS has applied the state and territory distribution for short-term visitors departing Australia who were in Australia for between six and twelve months. The state and territory distributions used for revised NOM estimates (shown in Table 6) are still subject to revision. The ABS expects that these estimates will improve as investigations proceed, and as actual data on state or territory of stay becomes available for this segment of the overseas visitor population (i.e. as outgoing passenger cards become available).
6. COMPONENTS OF NET OVERSEAS MIGRATION, States and territories - 2001-02
29 Due to changes in the methods used to adjust NOM estimates, caution should be used when comparing estimates over time. Table 7 describes the adjustment methods that have been applied to NOM estimates since September quarter 1996 (i.e. since the last intercensal period). Adjustments applied to overseas migration estimates will also be discussed in a special article in Migration, Australia, 2002-03 (cat.no.3412.0), scheduled for release on 28 April 2004.
7. MIGRATION ADJUSTMENT METHODS, September quarter 1996 to September quarter 2003
30 For further information on the measurement of net overseas migration, contact Rhonda de Vos on Canberra (02) 6252 6639, email firstname.lastname@example.org.
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