APPENDIX 3 ESTIMATING MIGRATION FOR SLAS


INTRODUCTION

A3.1 Post-censal estimates of the age and sex distributions of statistical local area (SLA) populations are made by updating the estimated resident population (ERP) by age and sex for the Census year using annual births, deaths and derived age and sex profiles of migration (both overseas and internal migration). While annual data on births (by sex) and deaths (by age and sex) are available for each SLA, data on migration into and out of SLAs (internal migration) for post-censal years are not available and are derived indirectly.


METHOD

Step 1 - Estimate net migration totals during year t to t+1 for each SLA

A3.2 First, net migration (i.e. the sum of overseas and internal migration) in an SLA during the year from t to t+1 is estimated based on the population at each endpoint and the number of births and deaths occurring during the year.

Step 2 - Derive components of net migration

A3.3 Next, net migration between t and t+1 is split into four components representing internal and overseas arrivals and departures. The following equation shows how these components contribute to net migration while the later equations illustrate how each component is estimated.

A3.4 Initial estimates of each component of net migration are calculated by multiplying the total population at time t+1 by a 'movement rate' specific for each component and calculated from one year ago SLA-level movement data from the most recent Census. For example, the internal arrival component would be estimated as:

A3.5 Internal departures (ID) and overseas arrivals and departures (OA and OD respectively) are calculated in the same way.

A3.6 Once an initial estimate for each of the four components of migration has been obtained (equation A3.3), a plus-minus iterative proportional fitting (IPF) procedure is used to satisfy both equation A3.2 and to ensure that when all SLAs in a state/territory are added, the four components equal the state/territory levels. Specifically, the difference between overseas arrivals and overseas departures, summed for all SLAs in a state/territory, must equal the net overseas migration estimate for the state/territory. Similarly, net internal migration summed across all SLAs in a state/territory must equal the net interstate migration estimate for the state/territory. The plus-minus IPF procedure is explained in Appendix 6 - The plus-minus proportional adjustment technique.


Step 3 - Determine age-sex profile for each component of migration

A3.7 The Census-based SLA age-sex profiles for overseas arrivals and departures and internal arrivals are prorated to the SLA OA, OD and IA totals for the year t to t+1 calculated in step 2 above.

A3.8 However SLA departures are a function of existing SLA population, and so Census-based age-sex specific departure rates are employed. As Census data excludes residents who have left Australia, the requisite data on usual address one year ago is only available for internal (i.e. inter-SLA) departures, not overseas departures.

A3.9 SLA age-sex internal departures for the year t to t+1 are obtained by multiplying the population for each single year of age and sex, who survived to the year being estimated, by the age-sex specific departure rate. Thus for each age and sex,

A3.10 ID' was initially calculated using data from the most recent Census (so c = the Census year) as follows:

A3.11 The departure rate in equation A3.6 is internal departures for the financial year prior to the Census as a rate of survived population at time c-1 (i.e. in the case of the 2006 Census, the 30 June 2006 population with migration removed). This can then be used in equation A3.5 to create, say, 2006-07 internal departures by applying it to the survived 2006 population.

A3.12 Once an initial age-sex estimate of IA, ID, OA and OD has been obtained for the year t+1, an IPF procedure is used to satisfy both the SLA migration component totals (equation A3.3) and to ensure that when all SLAs in a state/territory are added, the four components equal the state/territory age-sex levels. Specifically, for each age and sex, SLA OA and OD levels must total respective state/territory overseas migration levels, while net SLA internal migration (IA minus ID) must sum to each state/territory's net interstate migration.

A3.13 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 (small numbers in the denominator of IA', etc. can create improbable migration profiles).