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6.5 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'.
6.6 For further information on the improved methods see Information Paper: Improved Methods for Estimating Net Overseas Migration, 2006 (cat. no. 3107.0.55.003) and Information Paper: Statistical Implications of Improved Methods for Estimating Net Overseas Migration, Australia, 2007 (cat. no. 3107.0.55.005).
6.7 For information on the previous methods 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.
Estimating NOM with 12/16 rule
6.8 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.
6.9 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.
6.10 To measure a travellers actual duration of stay the ABS uses a unique personal identifier provided with the administrative data supplied by the Department of Immigration and Citizenship (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. For more information on the administrative data used see paragraph 9.62 in Chapter 9 - Data sources.
6.11 At the time preliminary estimates are required (5 to 6 months after the end of the reference quarter), the actual duration of stay in Australia (or overseas) for a traveller in the reference quarter is not known. Hence their contribution to NOM cannot be explicitly determined at this time using the 12/16 rule. Since full movement histories are not available within this timeframe, preliminary NOM estimates are therefore modelled on patterns of traveller behaviours observed in final NOM estimates for the same period two years earlier. More detailed information on preliminary NOM estimation is available later in this chapter.
Travellers vs movements
6.12 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.
6.13 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.