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Current methods for estimating NOM
For information on the reason for changing the method see paragraphs 21-27 of the Explanatory Notes. For detailed information on the methodology see the Information Paper: Improved Methods for Estimating Net Overseas Migration, Australia 2006 (cat. no. 3107.0.55.003) and the Information Paper: Statistical Implications of Improved Methods for Estimating Net Overseas Migration, Australia, 2007 (cat. no. 3107.0.55.005) or Chapter 6, Estimating Net Overseas Migration in Concepts, Sources and Methods, 2009 (cat. no. 3228.0.55.001).
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 full 16 months following a reference period. Currently, the final NOM estimates based on the '12/16 month rule' are considered to be of high quality.
Processing the final estimation of NOM provides, for each traveller in the reference quarter, a 'migration adjustment' based on their initial category of travel.
The initial category of travel has a key role in making preliminary estimates of NOM. It is determined by a number of dimensions:
For a visitor arriving and resident departing, it is their intended duration of stay that is recorded, whereas for a visitor departing and resident returning it is their measured duration of stay since the previous Australian border crossing that is recorded.
The 'initial category of travel' is also determined at the time of the movement. However, all travellers are assigned to one, and only one, category of travel during a reference quarter. Further information and a list of the ten categories of travel are detailed in the Glossary.
The migration adjustment, created during final NOM processing, is the difference between the traveller's true behaviour (actual duration of stay and its effect on ERP status) as recorded in final NOM and what was deduced from their initial category of travel. In other words, an adjustment is made to correct if they should have initially been counted in or out of the population for the reference quarter. The 'migration adjustment' is not required to determine final NOM estimates, but is essential for estimating future preliminary NOM.
Preliminary estimates of NOM are required within six months after the end of the reference quarter for the production of quarterly ERP of Australia and each of the states and territories. At that time, complete traveller histories for the 16 months following a reference quarter are not available.
To estimate preliminary NOM, the ABS developed a propensity model that uses the migration adjustments derived from final NOM during an earlier period. The migration adjustments are applied to travellers who are grouped according to their 'initial category of travel', age, country of citizenship and state or territory of usual/intended residence. The adjustment accounts for differences between a traveller's intended duration of stay and their actual duration of stay. The method is applied to each quarter and the preliminary estimate of annual NOM is the sum of the preliminary estimates for each quarter.
Preliminary NOM estimation is therefore modelled on patterns of traveller behaviours observed in final NOM during an earlier period. From September 2006 to June 2008 the migration adjustments used to estimate preliminary NOM were calculated from final NOM from the corresponding quarter two years earlier.