EXPLANATORY NOTES
INTRODUCTION
1 This product contains estimates of the resident population of Statistical Local Areas (SLAs) and Local Government Areas (LGAs) of Australia at 30 June 2004 and 30 June 2009, by age and sex.
2 To meet the conflicting demands for accuracy and timeliness there are several versions of sub-state/territory population estimates by age and sex. Preliminary estimates are normally available twelve months after the reference date, revised estimates the following year and rebased and final estimates after the following census. The estimates in this product are final for 2004 and preliminary for 2009, based on the final 2006 Census-based estimates.
ESTIMATED RESIDENT POPULATION - AUSTRALIA
3 Estimated resident populations (ERPs) are official estimates of the Australian population, which link people to a place of usual residence within Australia. Usual residence is that place where each person has lived or intends to live for six months or more from the reference date for data collection.
4 Estimates of the resident population are based on census counts by place of usual residence (excluding overseas visitors in Australia), with an allowance for net census undercount, to which are added the number of Australian residents estimated to have been temporarily overseas at the time of the census.
5 Population estimates for Australia and the states and territories are updated by adding to the estimated population at the beginning of each period the components of natural increase (births minus deaths, on a usual residence basis) and net overseas migration. For the states and territories, account is also taken of estimated interstate movements involving a change of usual residence.
6 After each census, estimates for the preceding intercensal period are finalised by incorporating an additional adjustment (intercensal discrepancy) to ensure that the difference between the ERPs at the two respective census dates agrees with the total intercensal change.
7 More detailed explanations of the concept of ERP, as adopted by the ABS for official population estimates, are contained in Information Paper: Population Concepts, 2008 (cat. no. 3107.0.55.006) and Population Estimates: Concepts, Sources and Methods (cat. no. 3228.0.55.001).
ESTIMATION OF SLA-BASED POPULATIONS
8 In Australia, the SLA is the base spatial unit used to collect and disseminate statistics other than those collected from the Population Censuses. In non-census years, the SLA is the smallest unit defined in the Australian Standard Geographical Classification (ASGC). In aggregate, SLAs cover the whole of Australia without gaps or overlaps. Populations for SLAs are estimated as at 30 June each year. Population estimates for LGAs and other regions are built up from SLA-level estimates.
9 The ERP as at census date for each SLA by age and sex is calculated based on usual residence census counts, excluding overseas visitors in Australia, with an allowance for net census undercount and the number of residents temporarily overseas (RTOs) at the census date. The estimates of net undercount are apportioned to SLAs based on age, sex, Indigenous status, state and territory, and (aside from the ACT) capital city/balance of state. The number of RTOs on census night is estimated based on coding addresses of residence to SLA from a sample of incoming passenger cards. For some areas, demographic adjustments are made. As the census is not held on 30 June (the 2006 Census was held on 8 August), further adjustments taking into account births, deaths and migration for the intervening period are made to obtain the ERP at 30 June. A procedure is then applied to avoid the release of unconfidentialised usual residence census counts while maintaining closeness to the unconfidentialised ERP.
10 For post-censal years, the absence of migration data at the SLA level means that it is not possible to estimate SLA populations by taking into account natural increase and net migration. Instead, these ERPs are calculated using a mathematical model, where relationships are established between changes in population and changes in indicators between the two most recent censuses for groups of SLAs. The choice of indicators varies across the states and territories, depending on availability and indicative ability, and includes dwelling approvals, Medicare enrolments and counts of people on the Australian Electoral Roll. Changes in these indicators are then used to estimate changes in the population of each area since the last census. In areas where indicator data is unreliable and migration can be assumed to be insignificant, population change since the previous census may be estimated by adding estimates of 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 scrutinised and validated by population analysts. Local knowledge, including that advised by local governments, may be used to adjust the figures for particular SLAs.
11 The total population estimates for each SLA for post-censal years are then broken down into age and sex components. The estimates by age and sex are updated from the previous year's estimates using registered births and deaths data, and synthetic estimates of migration based on the previous census. For areas where these data are deemed to be of insufficient quality, adjustments are made. While the output is presented by five-year age group (up to 80-84 years, then 85 years and over), all calculations are made at single year of age level (up to 84 years, then 85 years and over). Estimates at SLA level are constrained to state/territory population estimates.
12 LGA and SLA boundaries change occasionally. To enable the comparison of populations in different years, a time series of population estimates on consistent boundaries is prepared each year. The SLA estimates for all years back to the Census before the previous Census are reconstructed on SLA boundaries for the reference date, by age and sex. Indicator sources such as electoral roll counts are used in the calculation of these new historical estimates, with all new estimates constrained to previously-released estimates.
13 In census years, both preliminary total estimates (derived from updating the ERPs from the previous census) and rebased total estimates (based on the current census) are prepared. Differences between these two sets of estimates are referred to as intercensal errors. Rebased, or final, estimates of SLA populations for previous intercensal years are based on estimates derived by apportioning the intercensal error evenly across the five years, while constraining the SLA level estimates to state/territory estimates. For example, the rebased 2002 to 2005 estimates were derived by adding one-fifth of the 2006 intercensal error to the previous estimates of the 2002 population, two-fifths to the previous estimate of the 2003 population, and so on.
14 In recognition of the inherent inaccuracy involved in population estimation, population figures less than 1,000 in the summary text 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 accompanying this release, in order to facilitate calculations of sums, rates and percentages, accuracy to the last digit is not claimed and should not be assumed. Percentages mentioned in the summary text are calculated using unrounded numbers.
AUSTRALIAN STATISTICAL AREAS
15 This product contains data presented according to the Australian Standard Geographical Classification (ASGC) 2009 Edition, which refers to boundaries as defined at 1 July 2009. Under this classification, statistical areas are defined as follows:
- Local Government Areas (LGAs). These areas are the spatial units which represent the geographical areas of incorporated local government councils. The ABS has broadened the categories of legislation used to define local government areas for statistical purposes to include the Indigenous Council areas in the states and Northern Territory. The LGA Structure covers only incorporated areas of Australia, which are legally designated parts of states and territories over which incorporated local governing bodies have responsibility.
- Statistical Local Areas (SLAs). These geographical areas are, in most cases, identical with, or have been formed from a division of, whole LGAs. In other cases they represent unincorporated areas. In aggregate, SLAs cover the whole of a state or territory without gaps or overlaps. In some cases legal LGAs overlap Statistical Subdivision boundaries and therefore comprise two or three SLAs (Part A, Part B and, if necessary, Part C).
- Statistical Subdivisions (SSDs). These are of intermediate size, between SLAs and SDs. In aggregate, they cover the whole of Australia without gaps or overlaps. They are defined as socially and economically homogeneous regions characterised by identifiable links between the inhabitants. In the non-urban areas an SSD is characterised by identifiable links between the economic units within the region, under the unifying influence of one or more major towns or cities.
- Statistical Divisions (SDs). These consist of one or more SSDs. The divisions are designed to be relatively homogeneous regions characterised by identifiable social and economic units within the region, under the unifying influence of one or more major towns or cities. There were some major changes made to the SD structure in south-east Queensland in the 2006 edition of the ASGC.
- Statistical Districts. These consist of selected, significant, predominantly urban areas in Australia which are not located within a Capital City SD. Statistical Districts enable comparable statistics to be produced about these selected urban areas.
16 LGAs are proclaimed by various state and territory government authorities and changes are gazetted throughout the year. Presently, LGAs are used as the base on which SLAs are defined for the ASGC. Because this definition process takes time, some LGAs gazetted during the year leading up to an ASGC edition are not processed in time for inclusion in that edition, and are instead included in a later edition.
17 Further information about statistical areas is contained in
Australian Standard Geographical Classification (cat. no. 1216.0). This product also includes a complete series of SLA maps.
POPULATION PYRAMID GRAPHS
18 Population pyramids are used throughout this product to illustrate the age and sex distribution of a population. In each case, five-year age groups are represented on the vertical axis of the graph. Some of the population pyramids show the number, in thousands, of males on the left horizontal axis and of females on the right horizontal axis. Most of the population pyramids, however, plot a percentage distribution on the horizontal axes. In those cases, the population in a particular age-sex group in an area is expressed as a percentage of the total population in that area. Thus, the sum of the percentages indicated by the bars of one colour in a population pyramid will be 100% of the population of the area represented by that colour, e.g. the Sydney SD. Further, the sum of the percentages indicated by the bars of the other colour will be 100% of the population of the area represented by the other colour, e.g. the remainder of NSW. By using this method, the age and sex distribution of two areas can be compared irrespective of the relative sizes of the total populations of the areas.
ACKNOWLEDGEMENT
19 ABS products 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.
RELATED RELEASES
20 Other ABS releases that may be of interest to users of this product include:
21 Each issue of Population by Age and Sex, Regions of Australia contains the first release of the estimates of resident populations in SLAs and LGAs. Subsequently, the ERPs are updated several times, because of updates and revisions to the state and territory ERPs, changes to the ASGC and the availability of later Census data on which to rebase the ERPs. These multiple updates of the ERPs are not routinely released on the ABS web site, but can be obtained on request. The ABS maintains SLA and LGA ERP time series on consistent boundaries until two Censuses after a reference period and these can also be obtained on request.
22 As well as the statistics included in this and related products, additional information is available from the ABS website <
www.abs.gov.au> by selecting Topics @ a Glance, People and then Demography.
23 In addition to the demographic data that are freely available on the ABS website, more detailed data can be obtained on request. For example, the data in this product can be prepared for different age groups, including single year of age, and geographic areas. Updated and time series data, as described above, are also available. Contact the National Information and Referral Service on 1300 135 070 to discuss your population data needs and request an obligation-free quote.
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