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EXPLANATORY NOTES


INTRODUCTION

1 This release contains estimates of the resident population of Statistical Areas Level 2 to 4 (SA2s - SA4s) and Greater Capital City Statistical Areas (GCCSAs) of Australia. These estimates plus those for Local Government Areas (LGAs), Significant Urban Areas (SUAs), Remoteness Areas (RAs) and Electoral Divisions are also provided in the Downloads tab of this issue.

2 To meet the competing demands for accuracy and timeliness, there are several versions of sub-state population estimates. Preliminary estimates as at 30 June are normally available by April of the following year, revised estimates twelve months later and rebased and final estimates after the following Census. The estimates in this issue are final for 2006 to 2011, revised for 2012 to 2015, and preliminary for 2016.


ESTIMATED RESIDENT POPULATION - AUSTRALIA

3 Estimated resident population (ERP) is the official estimate of the Australian population, which links people to a place of usual residence within Australia. Usual residence within Australia refers to that address at which the person has lived or intends to live for six months or more in a given reference year. For the 30 June reference date, this refers to the calendar year around it.

4 Estimates of the resident population are based on Census counts by place of usual residence (excluding short-term overseas visitors in Australia), with an allowance for Census net undercount, to which are added the estimated number of Australian residents temporarily overseas at the time of the Census. A person is regarded as a usual resident if they have been (or expected to be) residing in Australia for a period of 12 months or more over a 16-month period.

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. After each Census, estimates for the preceding intercensal period are finalised (rebased) by incorporating an additional adjustment (intercensal difference) to ensure that the difference between the ERPs at the two respective Census dates agrees with the total intercensal change.

6 More detailed explanations of the concept of ERP, as adopted by the ABS for official population estimates, are contained in Information Paper: Population Concepts (cat. no. 3107.0.55.006) and Population Estimates: Concepts, Sources and Methods (cat. no. 3228.0.55.001).


ESTIMATION OF SUB-STATE POPULATIONS

7 In Australia, the SA2 (as defined in the Australian Statistical Geography Standard (ASGS)) is the base spatial unit used to collect and disseminate statistics other than those collected from the 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.

8 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 provided by the Department of Immigration and Border Protection. 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. A procedure is then applied to avoid the release of unconfidentialised usual residence Census counts while maintaining consistency to the unconfidentialised ERP.

9 For post-Census years, the absence of migration data at the SA2 level means that it is not possible to estimate SA2 populations by taking into account natural increase and net migration. Instead, ERP for most SA2s is 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 are 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 scrutinised and validated by ABS analysts. Local knowledge, such as that advised by local and state governments (including peer reviewers) is considered and used to adjust the figures for particular SA2s. Estimates at the SA2 level are constrained so that they add to the relevant state/territory population estimates.

10 To enable the comparison of regional populations over time, historical population estimates based on consistent updated geographic boundaries are prepared. These estimates correspond with previously released estimates (on different boundaries) where possible. When official statistical boundaries, such as Local Government Areas, are updated, historical estimates are prepared based on the updated boundaries.

11 In Census years, both preliminary estimates (derived from updating ERP 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 differences. Rebased estimates of SA2 populations for previous intercensal years are based on estimates derived by apportioning the intercensal difference evenly across the five years, while constraining the SA2 level estimates so that they sum to state/territory estimates. Rebased 2007 to 2010 estimates were derived by adding one-fifth of the 2011 intercensal difference to the previous estimate of the 2007 population, two-fifths to the previous estimate of the 2008 population, and so on.


ACCURACY OF SUB-STATE POPULATION ESTIMATES

12 An indication of the accuracy of ERPs can be gauged by assessing the size and direction of the intercensal differences. For Australia, the preliminary (unrebased) June 2011 ERP over-estimated the final rebased June 2011 ERP by 1.2% (278,300 people). For the states and territories, the 2011 intercensal differences ranged from -0.6% (Australian Capital Territory) to +2.3% (Queensland).

13 Summary statistics of the absolute values of these differences 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 unrebased 2011 estimates was prepared, updated from 2006 Census-based estimates and constrained so that they summed to final rebased state/territory ERP. These were compared with the final rebased 2011 SA2 estimates. The average absolute value of the intercensal differences for this series of 2011 SA2 estimates (excluding areas with less than 1,000 people) was 3.5%.

14 Average absolute intercensal differences 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.

Average absolute intercensal difference, Australia - 30 June 2011
	Number of SA2s	Average absolute intercensal difference
Size of SA2 (people)	no.	%
1,000 to 3,000	99	5.9
3,000 to 4,999	335	4.3
5,000 to 6,999	327	3.9
7,000 to 9,999	383	3.5
10,000 to 14,999	414	3.2
15,000 to 19,999	292	2.7
20,000 and over	224	2.3

• Statistical Areas Level 2 (SA2s). SA2s are medium-sized general purpose areas which aim to represent communities that interact together socially and economically. SA2s are based on officially gazetted suburbs and localities. In urban areas SA2s largely conform to one or more whole suburbs, while in rural areas they generally define the functional zone of a regional centre.
• Statistical Areas Level 3 (SA3s). SA3s are aggregations of whole SA2s and reflect a combination of widely recognised informal regions as well as administrative regions such as state government regions in rural areas and Local Government Areas in urban areas.
• Statistical Areas Level 4 (SA4s). SA4s are made up of whole SA3s and are designed to reflect labour markets. In rural areas, SA4s generally represent aggregations of small labour markets with socioeconomic connections or similar industry characteristics. Large regional city labour markets are generally defined by a single SA4. Within major metropolitan labour markets SA4s represent sub-labour markets.
• Greater Capital City Statistical Areas (GCCSAs). GCCSAs are built from whole SA4s and represent a broad socioeconomic definition of each of the eight state and territory capital cities. They contain not only the urban area of the city, but also the surrounding and non-urban areas where much of the population has strong links to the capital city, through for example, commuting to work.
• Remoteness Areas (RAs). RAs represent an aggregation of non-contiguous geographical areas which share common characteristics of remoteness. The delimitation criteria for RAs are based on the Accessibility/Remoteness Index of Australia (ARIA+), which measures the remoteness of a point based on the road distance to the nearest urban centre. The RA categories range from Major Cities to Very Remote. Each RA is created from a grouping of SA1s which have a particular degree of remoteness.
• Significant Urban Areas (SUAs). SUAs are aggregations of whole SA2s which represent concentrations of urban development with populations of 10,000 people or more. They do not necessarily represent a single Urban Centre, as they can represent a cluster of related Urban Centres with a core urban population over 10,000 people. They can also include related peri-urban and satellite development and the area into which the urban development is likely to expand. SUAs may cross a state or territory border.
• State Electoral Divisions (SEDs). An SED is an area legally prescribed for returning one or more members to the State or Territory Lower Houses of Parliament. Data for SEDs are approximated by aggregating the data for Statistical Areas Level 1 (SA1) that best fit the area.

• Commonwealth Electoral Divisions (CEDs). A CED is an area legally prescribed for returning one member to the House of Representatives, Australia's Federal Lower House of Parliament. Data for CEDs are approximated by aggregating the data for SA1s that best fit the area.

• Local Government Areas (LGAs). LGAs are ABS approximations of officially gazetted LGAs as defined by each state and territory local government departments. LGAs cover incorporated areas of Australia, which are legally designated areas for which incorporated local governing bodies have responsibility. The ABS updates LGAs annually, and prepares updated and historical population estimates based on these updated boundaries.

• ESRI Grid format which can only be opened in a Geographic Information System (GIS).
• GeoTIFF format is a Tagged Image File Format (TIFF). It is a raster graphics file format that is widely supported by graphics software. The Geo extension to the TIFF format is a metadata storage format which allows georeferencing information (datums, ellipsoid, coordinate systems, map projection) to be embedded within the TIFF file. These metadata allows Geographical Information Systems (GIS) software, such as MapInfo, ArcGIS or QGIS, to correctly interpret the location of the image and compare the image with other spatial referenced data.
• PNG format is a Portable Network Graphics File (PNG). It is a raster graphics file format that is widely supported by graphics software including those bundled with the major operating systems (Microsoft Windows, Apple OS X & iOS). The objective of publishing in PNG format is to allow users to quickly visualise a "picture" of these data.