TECHNICAL NOTE MEASURING NET OVERSEAS MIGRATION
BACKGROUND
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:
P(t) = the estimated resident population at time point t
P(t+1) = the estimated resident population at time point t+1
B = the number of births occurring between t and t+1
D = the number of deaths occurring between t and t+1
NOM = net overseas migration occurring between t and t+1.
2 Net overseas migration accounts for around half of population growth at the national level. This note outlines how the ABS calculates NOM estimates, including adjustments made to overcome some limitations of existing migration data.
3 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:
- permanent movement;
- long-term (one year or more) movement;
- short-term (less than one year) movement.
4 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).
5 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.
6 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 DIMA. 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.
MIGRATION ADJUSTMENTS
7 The ABS applies a number of adjustments to overseas arrivals and departures data in order to produce estimates of NOM. These mainly comprise adjustments designed to reflect differences between stated travel intentions and actual travel behaviour, but (in the case of revised NOM estimates) also include adjustments to transform numbers of overseas movements into numbers of travellers. These adjustments are collectively referred to as 'migration adjustments', although they have also been referred to in the past as 'category jumping' adjustments.
8 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 (cat. no. 3137.0) (<https://www.abs.gov.au>, click Themes, under People click on Demography, then ABS Demography Working Papers). In order to conduct such a comparison, data for a 15 month 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.
9 The following table describes the impact that various types of migration adjustments have on NOM estimates.
MIGRATION ADJUSTMENTS
10 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:
- long-term overseas visitors who stayed in Australia for less than 12 months (i.e. long-term visitors who stayed in Australia short-term);
- short-term overseas visitors who stayed in Australia for 12 months or more (i.e. short-term visitors who stayed in Australia long-term);
- Australian residents departing long-term who stayed overseas for less than 12 months (i.e. long-term departures who stayed overseas short-term);
- Australian residents departing short-term who stayed overseas for 12 months or more (i.e. short-term departures who stayed overseas long-term).
11 To reflect historical movement patterns, records with a reported intended duration of exactly one year are allocated to short-term and long-term on a pro-rata basis. For visitors, 75% of such records are allocated to short-term and 25% to long-term. The ratio is 67:33 for residents departing Australia.
12 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. The following table shows the proportion of long-term and short-term travellers in 2003-04 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 resident departures and short-term resident departures).
Changes in travel behaviour(a), Selected categories of movement(b) - September quarter 2003 to June quarter 2004 |
|  |
| | Long-term | Short-term | |
| | Arrivals | Departures | Arrivals | Departures | |
| | % | % | % | % | |
| |
| 2003 | | | | | |
| September | 69.9 | 49.3 | 2.8 | 2.8 | |
| December | 65.8 | 48.9 | 2.3 | 2.6 | |
| 2004 | | | | | |
| March | 70.3 | 51.7 | 3.3 | 3.3 | |
| June | 68.6 | 49.1 | 2.4 | 2.3 | |
| Average | 68.7 | 49.8 | 2.7 | 2.7 | |
| |
| (a) Proportion of travellers whose actual duration of stay or absence differed from their stated intentions. |
| (b) Based on stated intentions. |
13 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 2005 assumed that, based on the 2003-04 evidence, 68.7% of long-term visitor arrivals during the quarter would in fact stay in Australia for less than 12 months, while 49.8% of long-term resident departures would return to Australia within 12 months.
REVISED NOM ESTIMATES
14 Preliminary estimates of NOM for a financial year are usually revised in the following March quarter 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 revised 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:
- permanent (settler) arrivals who arrived in and left Australia in the same quarter, and did not return at any point during the 12 months following this arrival;
- permanent departures who left and returned to Australia in the same quarter, and did not depart at any point during the 12 months following this departure.
15 Migration adjustments applied to revised NOM estimates also adjust for multiple movements of travellers (i.e. converting numbers of movements into numbers of persons).
CHANGES TO MIGRATION ADJUSTMENT METHODS
16 Due to changes in the methods used to adjust NOM estimates, caution should be used when comparing estimates over time. The table below describes the adjustment methods that have been applied to NOM estimates since September quarter 1982. Adjustments were also discussed in a special article in the special article "Adjustments to overseas migration estimates" in the 2002-03 issue of this publication.
MIGRATION ADJUSTMENT METHODS - September quarter 1982 to June quarter 2005 |
| |
| Period | Adjustment method | |
| |
| September 1982 - June 1997 | (a)Category jumping' adjustments applied using previous methodology | |
| September 1997 - June 2001 | No adjustments applied (i.e. 'category jumping' set to zero) | |
| September 2001 - June 2004 | Current migration adjustments used (revised NOM estimates) | |
| September 2004 - June 2005 | Current migration adjustments methods used (preliminary NOM estimates) | |
| |
| (a) For further information, refer to Appendix 3 in Demographic Estimates and Projections: Concepts, Sources and Methods (cat. no. 3228.0). |
FURTHER INFORMATION
17 For further information on the measurement of net overseas migration, contact Phil Browning on Canberra (02) 6252 6639, email <phil.browning@abs.gov.au>.
Print Page
Print All