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3.21. Post-censal estimates of the age and sex distributions of SLA populations are made by updating the population by age and sex for the census year using annual births (by sex), deaths (by age and sex) and derived age and sex profiles of migration.
3.23. The estimate of total population growth for each SLA (see above) for the twelve months is split into natural increase and net migration components. Natural increase is derived for each SLA from birth and death registration statistics. Net migration is derived for each SLA as the difference between total population growth and natural increase. Net migration is then split into internal and overseas migration components. This is done by apportioning according to the relative contributions based on 1996 Census one-year migration data, in conjunction with State-level estimates of interstate migration and passenger-card-derived overseas migration.
3.25. These SLA age/sex arrival and departure profiles were then constrained so that for each age and sex the net effect across all SLAs in a State equalled the finalised 1995-96 interstate migration estimate.
3.26. An SLA's age/sex overseas arrivals profile was derived from 1996 Census counts for that SLA of people whose usual residence one year ago was overseas. The departure profile for each SLA is assumed to be the same as the arrival profile in the absence of age/sex data on overseas departures at the SLA level from either the census or outgoing passenger cards. For arrivals, the total of all SLAs within a State is constrained to the age/sex profile of permanent and long-term arrivals for 1995-96 for the State. For departures, the total of all SLAs within a State is constrained to the age/sex profile of permanent and long-term departures for 1995-96 for the State.
Procedure 1 - Estimation of net migration totals during year t to t+1 for each SLA
3.29. This step requires population estimates for each SLA as at year t and t+1, and recorded births and deaths occurring during the period t to t+1.
Procedure 2 - Split of SLA net migration into four components
3.30. Net migration obtained in equation (1) is then split into internal arrivals and departures and overseas arrivals and departures. Thus,
3.31. The first estimate of the four components of migration is obtained by multiplying the total population at time t+1 by a 'movement rate' specific for each component and calculated from 1996 Census movement data for SLAs for the period 1995-96. Thus,
Procedure 3 - Determination of the age-sex profile for each component of migration
3.33. The census-based SLA age-sex profiles for overseas arrivals and departures and internal arrivals [see paragraph 3.26] are prorated to the SLA OA, OD and IA totals for the year t to t+1 calculated in the previous step.
3.36. was initially calculated using 1996 Census-based data (so, t = 1996) as follows:
3.38. Once an initial age-sex estimate of IA, ID, OA and OD has been obtained for year t+1, an IPF procedure is used to satisfy both the SLA migration component totals [equation (3)] and to ensure that when all SLAs in a State are added, the four components equal the State age-sex levels. Specifically, for each age and sex, SLA OA and OD levels must total respective State overseas migration levels, while net SLA internal migration (IA minus ID) must sum to each State's net interstate migration.
3.39. At the conclusion of each annual iteration of the SLA age-sex estimation process, the SLA-specific OA, OD, IA and ID age-sex migration profiles are refined to correct unsustainable migration patterns.
Procedure 4 - Age-sex SLA population estimates
3.40. Having established estimates of the migration component, the census date population estimates for each SLA by age and sex are then updated in the normal way, (ie. after converting to financial year of birth, when necessary - by adding births, subtracting deaths and adding/subtracting migration). A more detailed account of this procedure at the national/State level is given in Chapter 2.