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8 In July 2011, the ABS replaced the nation's official statistical geography, the Australian Standard Geographical Classification (ASGC) with the new Australian Statistical Geography Standard (ASGS). To assist users in the transition to the ASGS, population estimates based on both the old and new geographies are contained within this issue.
9 In Australia, the SA2 (as defined in the ASGS) is now the base spatial unit used to collect and disseminate statistics other than those collected from the Population Census. In aggregate, SA2s cover the whole of Australia without gaps or overlaps. Populations for SA2s are estimated as at 30 June each year. Population estimates for larger regions are built up from SA2-level estimates. Under the ASGC, the base spatial unit for estimating population was the SLA.
10 The ERP as at census date for each SA2 is calculated based on usual residence census counts, excluding short-term overseas visitors in Australia, with an allowance for census net undercount and the number of residents temporarily overseas (RTOs) at the census date. The estimates of net undercount are apportioned to SA2s based on age, sex, Indigenous status, state and territory, and broad region. The number of RTOs on census night is estimated based on coding addresses of residence to SA2 from a sample of incoming passenger cards. As the census is not held on 30 June (the 2011 Census was held on 9 August), further adjustments taking into account births, deaths and migration for the intervening period are made to obtain the ERP at 30 June.
11 For post-census years, the absence of migration data at the SA2 level means that it will not be possible to estimate SA2 populations by taking into account natural increase and net migration. Instead, ERPs for most SA2s will be calculated using a mathematical model, where relationships are established between changes in population and changes in indicator data between the two most recent censuses for groups of SA2s. Current indicators include dwelling approvals, Medicare enrolments and counts of people on the Australian Electoral Roll. Changes in these indicators will be used to estimate changes in the population of each area since the last census. In areas where indicator data is unreliable or migration can be assumed to be insignificant, population change may be estimated by adding natural increase (births minus deaths) since the previous census. In some very small areas population change since the previous census may be assumed to be zero in the absence of reliable indicator data for these areas. All estimates are be scrutinised and validated by ABS analysts. Local knowledge, including that advised by local governments, may be used to adjust the figures for particular SA2s. Estimates at SA2 level are constrained to state/territory population estimates. Under the ASGC, this method was applied at the SLA level.
12 To enable the comparison of regional populations over time, historical population estimates based on consistent updated boundaries are prepared. These estimates correspond with previously-released estimates (on different boundaries) where possible. When official statistical boundaries, such as for Local Government Areas, are updated in future, historical estimates will be prepared based on the updated boundaries.
13 In census years, both preliminary estimates (derived from updating the ERPs from the previous census) and rebased estimates (based on the current census) are prepared. Differences between these two sets of estimates are referred to as intercensal errors. Rebased estimates of SA2 populations for previous intercensal years are based on estimates derived by apportioning the intercensal error evenly across the five years, while constraining the SA2 level estimates to state/territory estimates. Rebased 2007 to 2010 estimates were derived by adding one-fifth of the 2011 intercensal error to the previous estimates of the 2007 population, two-fifths to the previous estimate of the 2008 population, and so on.
ACCURACY OF SUB-STATE POPULATION ESTIMATES
14 An indication of the accuracy of ERPs can be gauged by assessing the size and direction of the intercensal errors. For Australia, the preliminary June 2011 ERP over-estimated the preliminary rebased June 2011 ERP by 1.32% (294,400 people). For the states and territories, the 2011 intercensal errors ranged from -0.57% (Australian Capital Territory) to +2.37% (Queensland).
15 Summary statistics of the absolute values of these errors can be used to assess the accuracy of a number of population estimates. To give an indication of the quality of SA2-based estimates, a set of experimental estimates was prepared, updated from 2006 census-based estimates, and compared with the preliminary rebased 2011 SA2 estimates. The average absolute value of the intercensal errors for this experimental series of 2011 SA2 estimates (excluding areas with less than 2000 people) was 4.0%.
16 Average absolute intercensal errors for the 2011 SA2 experimental estimates decreased with increasing population size; that is, SA2s with large populations recorded the smallest percentage errors while small SA2s had the largest percentage errors.
17 In recognition of the inherent inaccuracy involved in estimating population, population figures in text and accompanying summary tables published by the ABS are generally rounded. In this commentary for this product, figures less than 1,000 are rounded to the nearest ten, figures over 1,000 are rounded to the nearest hundred, and figures over 1 million are rounded to the nearest 10,000 or 100,000. While unrounded figures are provided in the spreadsheets, accuracy to the last digit is not claimed and should not be assumed. Estimates of change in population are based on unrounded numbers.
18 The Statistical Area Level 1 (SA1) is the smallest unit for the release of Census data. There are approximately 55,000 SA1s and they cover the whole of Australia without gaps or overlaps. To provide some indication of estimated resident population below the SA2 level, the ABS prepares population estimates for SA1s. These estimates can be aggregated to form population estimates for regions such as State Suburbs and Electoral Divisions. By this means, population estimates for areas other than those provided in this product may be available on request.
19 Population estimates at the SA1 level as at 30 June of a census year are compiled by apportioning the population estimate for each SA2 across the SA1s within the SA2, using census usual residence counts. In post-census years, the 30 June population estimates for SA2s will be apportioned across SA1s by taking into account population change implied by indicator data at the SA1-level in the years following the Census. Under the ASGC, this method was applied to produce estimates at the Collection District (CD) Level.
LOCAL GOVERNMENT AREA POPULATIONS
20 Population estimates for ASGC2011-based Statistical Local Areas (SLAs) that can be built from whole SA2s were produced by aggregating the SA2-level estimates accordingly. SA1-based ERP formed the basis for preparing estimates for the remaining SLAs. Estimates for SLAs that are aggregates from whole SA1s were produced by aggregating the estimates for those SA1s. Where SLAs crossed SA1 boundaries, estimates of the share of the SA1 population that resided in SLAs were made by converting the SA1 population across those SLAs, based on census counts aggregated from mesh block level, and whole and partial SA1 estimates were aggregated accordingly to SLA level. In post census years, estimates for Local Government Areas (LGAs) will be prepared in this way, using updated estimates of SA1 to LGA splits. As ASGS-based LGAs can be built up from ASGC2011-based SLAs, population estimates for LGAs based on the 2011 edition of the ASGS were built up from these SLA-level estimates.
AUSTRALIAN STATISTICAL AREAS
21 This publication contains data presented according to the 2011 edition of the Australian Statistical Geography Standard (ASGS), which refers to boundaries as defined at 1 July 2011. Under this classification, statistical areas are defined as follows:
22 Remoteness Area ERP included in this issue is provided initially based on the 2006 edition of the ASGC Remoteness Structure only. The 2011 ASGS-based Remoteness Structure, calculated using results of the 2011 Census, is scheduled for release in late 2012. ERP by ASGS-based Remoteness Areas will then be added to the Downloads tab of this issue.
23 Further information on these statistical areas is contained in Australian Statistical Geography Standard: Volume 1 - Main Structure and Greater Capital City Statistical Areas, July 2011 (cat. no. 1270.0.55.001) and Australian Statistical Geography Standard: Volume 3 - Non ABS Structures, July 2011 (cat. no. 1270.0.55.003).
24 A complete series of SA2 maps is available in Australian Statistical Geography Standard: Volume 1 - Main Structure and Greater Capital City Statistical Areas, July 2011 (cat. no. 1270.0.55.001).
AVERAGE ANNUAL RATE OF GROWTH
25 The average annual growth rate is calculated as a percentage using the formula below, where P0 is the population at the start of the period, Pn is the population at the end of the period and n is the length of the period between P0 and Pn in years.
RANKING POPULATION CHANGE
26 This product ranks regions according to both ‘largest’ and ‘fastest’ growth, identifying areas with significant changes in population. Largest growth is based on the absolute change in population between June 2001 and June 2011, while fastest growth is based on the rate of change in population (expressed as a percentage). Regions with populations of less than 1,000 people at June 2011 have been excluded from the fastest growth rankings.
27 Due to the inherent imprecision of regional population estimates and variation in population size, rankings should be considered indicative of relative growth between regions, not definitive.
CENTRE OF POPULATION
28 The centre of population is a measure used to describe the spatial distribution of a population. The method used to calculate centres of population in this product is based on the centroid and population of each SA1. To calculate the centre of population for an area, the latitude and longitude coordinates of the centroid of each SA1 in that area are multiplied by the SA1's estimated resident population to obtain weighted latitudes and longitudes for each SA1. These are summed to obtain a weighted latitude and longitude coordinate for the area, then divided by the total population of the area to obtain a single latitude and longitude coordinate.
29 Due to the inherent imprecision in small area estimates, the centre of population should be considered indicative only of the distribution of population, and cannot be ascribed to an exact location. The use of different geographical level data could result in different centres of population.
CALCULATION OF AREAS AND POPULATION DENSITY
30 The area figures used in this issue are based upon the SA2 level of the Australian Statistical Geography Standard, 2011 edition. The areas of the SA2s were calculated using ABS standard Geographic Information Systems software from the digital boundaries of this ASGS edition. Higher level spatial unit area figures are aggregations of the relevant SA2 areas. These areas are included in the ASGS-based spreadsheet accompanying this publication.
31 The population density for a region is calculated by dividing the estimated resident population for the region by its area in square kilometres. The result is expressed as a number of people per square kilometre. The population densities of SA2s are illustrated in the Population Density map in this product. This map should be considered an approximation of the true distribution of the population. For example, an SA2 comprising one or more small population centres surrounded by a large, sparsely populated area will be illustrated with the average population density for the SA2 as a whole, so the small population centres will not be apparent.
32 ABS publications draw extensively on information provided freely by individuals, businesses, governments and other organisations. Their continued cooperation is very much appreciated; without it, the wide range of statistics published by the ABS would not be available. Information received by the ABS is treated in strict confidence as required by the Census and Statistics Act 1905.
33 Other ABS releases that are freely available on the ABS website <www.abs.gov.au> and may be of interest to users of this product include:
Information Paper: Ensuring the Quality of Rebased Population Estimates, 2011 (cat. no. 3250.0)
Population by Age and Sex, Regions of Australia (cat. no. 3235.0)
Australian Demographic Statistics (cat. no. 3101.0)
Australian Historical Population Statistics (cat. no. 3105.0.65.001)
Births, Australia (cat. no. 3301.0) - SLA-specific datasets
Deaths, Australia (cat. no. 3302.0) - SLA-specific datasets
Regional Statistics - available on the ABS home page
Population Estimates: Concepts, Sources and Methods (cat. no. 3228.0.55.001)
Information Paper: Rebasing Population Estimates, Australia, 2011 (cat. no. 3101.0.55.001)
Information Paper: Population Concepts, 2008 (cat. no. 3107.0.55.006)
Information Paper: Aboriginal and Torres Strait Islander Demographic Statistics Work Program and Release Plans, Apr 2012 (cat. no. 3238.0.55.003)
ADDITIONAL STATISTICS AVAILABLE
34 As well as the statistics included in this and related publications, additional information is available from the ABS website <www.abs.gov.au> by selecting Topics @ a Glance, People and then Demography.
35 The ABS may have other relevant data available on request. Inquiries should be made to the National Information and Referral Service on 1300 135 070.
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