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# National, state and territory population methodology

Reference period
December 2019
Released
18/06/2020

## Explanatory notes

### Introduction

1 This quarterly release contains the most recent estimates of the resident populations (ERP) of Australia and the states and territories based on the results of the 2016 Census of Population and Housing held on 9 August 2016 (with various adjustments described in paragraphs 5 and 6), and the addition of quarterly components of population growth. The ABS has used the 2016 Census to produce final rebased estimates of the resident population (refer to paragraph 6). This release contains the latest available statistics on births, deaths (including infant deaths) and overseas and interstate migration. In addition, the release includes estimates of the resident population by age groups and major population regions. Articles on specific demographic topics will be released on the ABS web site in conjunction with this release.

2 Population estimates commencing from September quarter 1993 include estimates for another category of the state and territory level, known as Other Territories. Other Territories include Jervis Bay Territory (previously included with the Australian Capital Territory), as well as Christmas Island and the Cocos (Keeling) Islands which were previously excluded from population estimates for Australia. From 1 July 2016 Norfolk Island has been included in the Other Territory category following the introduction of the Norfolk Island Legislation Amendment Act 2015. Data for Other and External Territories are detailed separately in table 3.

3 Estimates for Australian External Territories will be updated annually as at 30 June unless a more recent estimate is required for electoral apportionment purposes under the Commonwealth Electoral Act 1918.

4 Australia’s population estimates for the period since 1971 are compiled according to the place of usual residence of the population. An explanation of the place of usual residence conceptual basis for population estimates is given in Information Paper: Population Concepts (cat. no. 3107.0.55.006) and also in Population Estimates: Concepts, Sources and Methods (cat. no. 3228.0.55.001).

### Method of estimation

5 Estimates of the resident population are based on Census counts by place of usual residence, to which are added the estimated Census net undercount and the number of Australian residents estimated to have been temporarily overseas on Census night. Overseas visitors in Australia on Census night are excluded in this calculation. Post-Census ERP is obtained by adding to the estimated population at the beginning of each period the component of natural increase (on a usual residence basis) and the component of net overseas migration. For the states and territories, estimated interstate movements involving a change of usual residence are also taken into account.

6 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 30 June dates in the respective Census years agrees with the total intercensal change.

### Status of quarterly ERP data

7 The status of quarterly ERP data changes over time from preliminary to revised to final as new component data becomes available. ERP is updated most quarters due to revisions to the component data for earlier quarters, but will only change status to revised once both natural increase and net overseas migration for that quarter have a status of revised. The table below shows the current status of ERP and the components of population change: natural increase, overseas migration and interstate migration.

### Status of quarterly Estimated Resident Population (ERP) data, as at 18 June 2020

Census baseNatural increaseOverseas migrationInterstate migrationERP STATUS
Sep.1991-Jun. 2016Based to 1996, 2001, 2006, 2011 &
2016 Censuses as applicable
FinalFinalFinalFINAL
Sep. 2016-Jun. 20182016 CensusRevised - based on date of occurrenceFinal - based on actual traveller behaviourPreliminary - based on modelled expansion factors from 2016 CensusREVISED
Sep. 2018-Dec. 20182016 CensusPreliminary - based on date of registrationFinal - based on actual traveller behaviourPreliminary - based on modelled expansion factors from 2016 CensusPRELIMINARY - updated due to revised component data
Mar. 2019-Sep. 20192016 CensusPreliminary - based on date of registrationPreliminary - based on modelled traveller behaviourPreliminary - based on modelled expansion factors from 2016 CensusPRELIMINARY - updated due to revised component data
Dec. 20192016 CensusPreliminary - based on date of registrationPreliminary - based on modelled traveller behaviourPreliminary - based on modelled expansion factors from 2016 CensusPRELIMINARY

### Natural increase: births and deaths

8 Natural increase is a major component of ABS quarterly state and territory population estimates and is calculated using the estimated number of births and deaths. The births and deaths data in this release are shown by state and territory of usual residence, using year/quarter of registration for preliminary data and year/quarter of occurrence for both revised and final data. This may affect time series comparisons within relevant tables.

9 The timeliness and accuracy of ABS quarterly population estimates depend in part on the timeliness and accuracy of estimates of births and deaths which are based on registrations. To provide timely estimates, the ABS produces preliminary estimates using births and deaths by quarter of registration as a proxy for quarter of occurrence. For revised estimates, a factor has been applied to the number of occurrences to allow for those occurrences which were yet to be registered at the time of revision. The major difficulty in this area is that while the vast majority of births and deaths are registered promptly, a small proportion of registrations are delayed for months or even years.

10 Preliminary birth and death estimates are subject to fluctuations caused by lags or accumulations in the reporting of births and deaths registrations. Accumulations can result from the eventual processing of lagged registrations in a later quarter. As a result, preliminary quarterly estimates can be an underestimate or an overestimate of the true numbers of births and deaths occurring in a reference period. Lags or accumulations in births and deaths registrations can be caused by:

• late notification of a birth or death event to a state or territory registry;
• delays arising from incomplete information supplied for a registration;
• procedural changes affecting the processing cycles in any of the state and territory registries; or
• resolution of issues that may arise within the ABS or registry processing systems.

11 Birth and death registration data contributing to preliminary estimates which are higher or lower than usual are noted below along with any explanations provided by the relevant state or territory registrars:

• December 2019: Victoria. As a result of joint investigations between the ABS and the Registry additional death registrations from 2017, 2018 and 2019 were identified that had not previously been provided to the ABS. An issue associated with the Registry's previous processing system (replaced in 2019) has resulted in delays to the provision of some death registrations to the ABS. Approximately 570 additional deaths registrations have now been included in the December 2019 ERP, with the deaths being added across the five previous quarters. The remaining additional deaths (approximately 2200) occurred in 2016-17 and 2017-18, and will be included as part of the final revision of intercensal components in June 2023.
• June 2019: Victoria. There are higher than usual numbers of births and deaths registrations as a result of catch up in processing from March 2019, see below.
• March 2019: Victoria. The Victorian Registry of Births, Deaths and Marriages implemented a new registration system in February 2019. This large scale system change caused processing delays which have led to lower than usual numbers of birth and death registrations in the March quarter 2019. The ABS has worked closely with the registry through this system change and anticipate that lagged registrations will be caught up through the remainder of 2019.
• March 2019: Australia. The delay in processing of birth and death registrations in Victoria is the main reason for low national birth and death counts.

### Overseas migration

12 For the purposes of NOM, a person is regarded as a usual resident if they have been (or expect to be) residing in Australia for a period of 12 months or more. These 12 months do not need to be continuous and are measured over a period of 16 months.

13 The ABS introduced the 12/16 month rule for calculating NOM in September quarter 2006. Consequently, this point marks a break in series and NOM estimates from earlier periods are not directly comparable. For further information on the 12/16 month rule see the Technical Note: '12/16 month rule' Methodology for Calculating Net Overseas Migration from September quarter 2006 onwards in Migration, Australia, 2008-09 (cat. no. 3412.0).

14 Preliminary estimates of NOM are required less than six months after the reference quarter for the production of quarterly estimates of the population of Australia and its states and territories. At that time, complete traveller histories for the 16 months following a reference quarter are not available and as a result preliminary estimates are produced.

15 Final NOM estimates are produced once data on overseas movements become available for the 16 months following the reference period. This enables each traveller’s actual duration of stay to be calculated and the 12/16 month rule to be fully applied. Final NOM estimation methods use a traveller's actual duration of stay in or out of Australia to determine their contribution to NOM estimates and consequently their inclusion or exclusion from the ERP.

16 In an initiative to create a more efficient and streamlined process for travellers departing Australia, the requirement for international travellers to complete an outgoing passenger card was removed by the Department of Home Affairs from 1 July 2017. Due to the removal of the card the ABS has reviewed its net overseas migration (NOM) statistics, methodology and processing systems. Historical NOM data from September quarter 2011 onwards has been produced based on the new methods and has been used in the 2011-2016 intercensal period as part of the final rebasing of population estimates. As NOM estimates cannot be finalised until 16 months after the reference period, NOM is subject to revision. Testing has shown that the revision between preliminary and final NOM using the new methodology has improved when compared to the previous method. For further information see the Information Paper: Improvements to estimation of net overseas migration (cat. no. 3412.0.55.004).

17 From the March quarter 2020, the COVID-19 pandemic caused extreme disruption to ordinarily expected travel patterns and volumes to and from Australia. The Australian Government placed restrictions on travel to and from Australia resulting in vast reductions in the volume of overseas movements. In order to mitigate any adverse effects these travel restrictions and resulting reductions may have on the preliminary NOM estimates, the ABS implemented a more frequent revision cycle. From December quarter 2019, all NOM estimates which have not already been made final shall be revised each quarterly processing cycle. These revisions will utilise all available overseas movements data in order to incorporate the most up to date information in the NOM estimates. Prior to this, due to the stability of travel patterns and the accuracy of the preliminary estimates, NOM estimates were ordinarily revised only once. This occurred one year after production of the preliminary estimate when they were made final.

18 Statistics on migration and related data are also published regularly by the Department of Home Affairs http://www.homeaffairs.gov.au.

### Diplomatic personnel

19 Australia's ERP includes all people, regardless of nationality or citizenship, who usually live in Australia, with the exception of foreign diplomatic personnel and their families. Foreign diplomatic personnel and their families were removed from overseas migration estimates from 1 July 2006. The previous methodology for estimating overseas migration was unable to exclude diplomatic personnel and their families.

### Interstate migration

20 Quarterly interstate migration cannot be directly measured and is estimated using administrative data. To do this the ABS uses information on interstate changes of address from Medicare records (produced by the Department of Human Services) and the Department of Defence in the case of the military.

21 The Medicare-based model is calibrated using migration data from the most recent Census (that data is available for), from which updated expansion factors are calculated. Expansion factors account for undercoverage of Medicare data by age and sex. The current model includes the following characteristics:

• Medicare data is lagged by three months (both for calculating expansion factors and for estimating progressive quarters of interstate migration);
• All single year of age data were smoothed to produce expansion factors;
• capping was applied to expansion factors; and
• expansion factors were applied to males aged 17 to 35 years and females aged 17 to 30 years (this differs to the age range used in the 2011-16 method).

22 The Medicare system theoretically covers all Australian citizens and permanent residents, as well as some temporary visa holders. However, some Australian usual residents do not access the Medicare system, such as temporary migrants or those who have access to other health services. One group is the military. Interstate defence force movements not covered by Medicare are estimated and then added to the Medicare-based interstate movement estimates. Quarterly counts of defence force personnel by age, sex and state/territory, supplied by the Department of Defence, form the number of interstate defence force movements, and 70% are assumed to be not covered by the Medicare-based estimates.

23 Preliminary interstate migration estimates for Other Territories are not available. These movements are included in the data for New South Wales (Jervis Bay Territory and Norfolk Island) and Western Australia (Christmas Island and Cocos (Keeling) Islands). Revised interstate migration estimates will become available following the 2021 Census.

24 For further information see Australian Demographic Statistics, December quarter 2017 (cat. no. 3101.0) Technical Note 2: 2016 Census update of the net interstate migration model.

### Confidentiality

25 The Census and Statistics Act, 1905 provides the authority for the ABS to collect statistical information, and requires that statistical output shall not be published or disseminated in a manner that is likely to enable the identification of a particular person or organisation. This requirement means that the ABS must take care and make assurances that any statistical information about individual respondents cannot be derived from published data.

26 Some techniques used to guard against identification or disclosure of confidential information in statistical tables are suppression of sensitive cells, and random adjustments to cells with very small values. To protect confidentiality within this release, some cell values may have been suppressed and are not available for publication (np) but included in totals where applicable. In these cases, data may not sum to totals due to the confidentialisation of individual cells.

### Rounding

27 In this release, population estimates and their components have sometimes been rounded. Where figures have been rounded, discrepancies may occur between sums of component items and totals.

### Acknowledgment

28 ABS statistics 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 in 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.

### Use of population estimates in reporting

29 The ABS provides regular explanatory information to support users in understanding both population trends and methodological changes, but does not comment on the specific use (or otherwise) of official population estimates by other organisations or individuals.

30 Population estimates are used extensively within the Australian community, including in a range of agreements. Although the ABS acknowledges that the official population estimates and changes in these estimates are specifically referenced in various agreements, it neither endorses nor disapproves of the terms of the agreements and decisions made by parties in relation to those agreements. In addition, the ABS does not provide a position on disputes arising from the interpretation of terms of an agreement that reference official population estimates.

31 As well as the statistics included in this and related products, 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.

## Glossary

### Show all

#### 12/16 month rule

Under a '12/16 month rule', incoming overseas travellers (who are not currently counted in the population) must be resident in Australia for a total period of 12 months or more, during the 16 month follow-up period to then be added to the estimated resident population. Similarly, those travellers departing Australia (who are currently counted in the population) must be absent from Australia for a total of 12 months or more during the 16 month follow-up period to then be subtracted from the estimated resident population.

The 12/16 month rule does not have to be continuous and takes account of those persons who may have left Australia briefly and returned, while still being resident for 12 months out of 16. Similarly, it takes account of Australians who live most of the time overseas but periodically return to Australia for short periods.

#### Average annual rate of growth

The average annual growth rate, r, is calculated as a percentage using the formula:

$$\large{r=\left[(\frac{P_n}{P_o})^{\frac{1}{n}}-1\right] \times 100}$$

where $$\text{P}_0$$ is the population at the start of the period, $$\text{P}_n$$ is the population at the end of the period and n is the length of the period between $$\text{P}_0$$ and $$\text{P}_n$$ in years.

#### Baby boomer

Refers to people born Post-World War II between the years 1946 and 1964.

#### Birth

The delivery of a child, irrespective of the duration of pregnancy, who, after being born, breathes or shows any other evidence of life such as heartbeat.

#### Census

The complete enumeration of a population at a point in time with respect to well-defined characteristics (e.g. Persons, Manufacturing, etc.). When the word is capitalised, "Census" refers to the national Census of Population and Housing.

#### Death

Death is the permanent disappearance of all evidence of life after birth has taken place. The definition excludes deaths prior to live birth.

For the purposes of the Deaths and Causes of Death collections compiled by the ABS, a death refers to any death which occurs in, or en route to Australia and is registered with a state or territory Registry of Births, Deaths and Marriages.

#### Estimated resident population (ERP)

The official measure of the population of Australia is based on the concept of usual residence. It refers to all people, regardless of nationality, citizenship or legal status, who usually live in Australia, with the exception of foreign diplomatic personnel and their families. It includes usual residents who are overseas for less than 12 months over a 16-month period. It excludes overseas visitors who are in Australia for less than 12 months over a 16-month period.

Estimates of the Australian resident population are generated on a quarterly basis by adding natural increase (the excess of births over deaths) and net overseas migration (NOM) occurring during the period to the population at the beginning of each period. This is known as the cohort component method, and can be represented by the following equation:

$$\text{P}_{t+1} = \text{P}_t + \text{B} - \text{D} + \text{NOM}$$ where:

$$\text{P}_{t}$$ = the estimated resident population at time point $$t$$

$$\text{P}_{t+1}$$ = the estimated resident population at time point $$t+1$$

$$\text{B}$$ = the number of births occurring between $$t$$ and $$t+1$$

$$\text{D}$$ = the number of deaths occurring between $$t$$ and $$t+1$$

$$\text{NOM}$$ = net overseas migration occurring between $$t$$ and $$t+1$$.

For state and territory population estimates, an additional term is added to the equation representing net interstate migration (NIM) occurring between $$t$$ and $$t+1$$, represented by the following equation:

$$\text{P}_{t+1} = \text{P}_t + \text{B} - \text{D} + \text{NOM} +\text{NIM}$$.

#### Greater Capital City Statistical Area (GCCSA)

Represent the socioeconomic area of each of the eight state and territory capital cities. These boundaries are built from aggregations of whole Statistical Areas Level 4. GCCSA boundaries represent a broad socioeconomic definition of each capital city, they contain not only the urban area of the capital city, but also surrounding and non-urban areas where much of the population has strong links to the capital city, through for example, commuting to work. For further information see Australian Statistical Geography Standard (ASGS): Volume 1 - Main Structure and Greater Statistical Areas, July 2016 (cat. no. 1270.0.55.001).

#### Infant death

An infant death is the death of a live-born child who dies before reaching their first birthday.

#### Infant mortality rate (IMR)

The number of deaths of children under one year of age in a financial year per 1,000 live births in the same financial year.

#### Intercensal difference

Intercensal difference is the difference between two estimates at 30 June of a Census year population: the first based on the latest Census, and the second arrived at by updating the 30 June estimate of the previous Census year with intercensal components of population change. It is caused by differences in the start and/or finish population estimates and/or in estimates of births, deaths or migration in the intervening period which cannot be attributed to a particular source. For further information see Population Estimates: Concepts, Sources and Methods (cat. no. 3228.0.55.001).

#### Median age

For any distribution, the median value is that which divides the relevant population into two equal parts, half falling below the value, and half exceeding it. Thus, the median age is the age at which half the population is older and half is younger.

Prior to September quarter 2006, the ABS applied a number of adjustments to overseas arrivals and departures data in order to produce estimates of net overseas migration (NOM). These mainly comprised adjustments designed to reflect differences between stated travel intentions and actual travel behaviour. Until recently, adjustments used by ABS to produce NOM estimates were collectively referred to as 'category jumping adjustments'. They are now referred to more simply as 'migration adjustments'.

#### Natural increase

The number of births minus the number of deaths.

#### Interstate migration

The movement of people over a state or territory boundary for the purpose of changing their place of usual residence. Net interstate migration is the number of arrivals minus the number of departures.

#### Net overseas migration (NOM)

Net overseas migration is the net gain or loss of population through immigration to Australia and emigration from Australia. Under the current method for estimating final net overseas migration this term is based on a traveller's actual duration of stay or absence using the '12/16 month rule'. Preliminary NOM estimates are modelled on patterns of traveller behaviours observed in final NOM estimates for the same period one year earlier. NOM is:

• based on an international traveller's duration of stay being in or out of Australia for 12 months or more over a 16-month period;
• the difference between:

• the number of incoming international travellers who stay in Australia for 12 months or more over a 16-month period, who are not currently counted within the population, and are then added to the population (Overseas migrant arrivals); and
• the number of outgoing international travellers (Australian residents and long-term visitors to Australia) who leave Australia for 12 months or more over a 16-month period, who are currently counted within the population, and are then subtracted from the population (Overseas migrant departures).

#### Overseas migrant arrivals

Overseas migrant arrivals (also referred to as NOM arrivals) are all overseas arrivals that contribute to net overseas migration (NOM). It is the number of incoming international travellers who stay in Australia for 12 months or more over a 16 month period, who are not currently counted within the population, and are then added to the population.

Under the current method for estimating final net overseas migration this term is based on a traveller's actual duration of stay using the '12/16 month rule'.

#### Overseas migrant departures

Overseas migrant departures (also referred to as NOM departures) are all overseas departures that contribute to net overseas migration (NOM). It is the number of outgoing international travellers (Australian residents and long term visitors to Australia) who leave Australia for 12 months or more over a 16 month period, who are currently counted within the population, and are then subtracted from the population.

Under the current method for estimating final net overseas migration this term is based on a traveller's actual duration of absence using the '12/16 month rule'.

#### Net undercount

The difference between the actual Census count (including imputations) and an estimate of the number of people who should have been counted in the Census. This estimate is based on the Post Enumeration Survey (PES) conducted after each Census. For a category of person (e.g. based on age, sex and state of usual residence), net undercount is the result of Census undercount, overcount, differences in classification between the PES and Census and imputation error.

#### Population growth

For Australia, population growth is the sum of natural increase and net overseas migration. For states and territories, population growth also includes net interstate migration. After the Census, intercensal population growth also includes an allowance for intercensal difference.

#### Population growth rate

Population change over a period as a proportion (percentage) of the population at the beginning of the period.

#### Post Enumeration Survey (PES)

The Census Post Enumeration Survey (PES) is a household survey conducted following the Census. The PES allows the ABS to estimate the number of people missed in the Census and the number counted more than once or in error. Historically more people are missed than are counted more than once in Australia, leading to a net undercount. Results from the PES contribute to a more accurate calculation of the estimated resident population (ERP) for Australia and the states and territories, which is then backdated to 30 June of the Census year.

#### Rebasing of population estimates

After each Census, the ABS uses Census counts by place of usual residence which are adjusted for undercount to construct a new base population figure for 30 June of the Census year. Because this new population estimate uses the Census as its main data source, it is said to be 'based' on that Census and is referred to as a population base.

Rebasing refers to the process by which the ABS uses this new base to update all previously published population estimates from the previous census to the most recent census (the intercensal period). For further information on rebasing to the 2016 Census see Australian Demographic Statistics, December quarter 2017 (cat. no. 3101.0) Feature Article: Final Rebasing of Australia's Population Estimates using the 2016 Census.

#### Recasting of population estimates

The recasting of ERP was a one-off process undertaken during the course of rebasing to the 2011 Census. The decision to recast historical ERP data from September 1991 to June 2006 was in response to the unusually high preliminary intercensal difference, resulting from a change in the methodology used to estimate undercount in the 2011 Census. For further information see Australian Demographic Statistics, December quarter 2012 (cat. no. 3101.0) Feature Article: Recasting 20 Years of ERP.

#### Sex ratio

The sex ratio relates to the number of males per 100 females. The sex ratio is defined for the total population, at birth, at death and among age groups by appropriately selecting the numerator and the denominator of the ratio.

#### Significant Urban Area (SUA)

Aggregations of whole Statistical Areas Level 2 to define and contain major urban and near-urban concentrations of over 10,000 people. They include the urban population, any immediately associated populations, and may incorporate together one or more closely associated Urban Centre and Localities and the areas between. They are designed to incorporate any likely growth over the next 20 years. SUAs do not cover the whole of Australia, and may cross State boundaries. For further information see Australian Statistical Geography Standard (ASGS): Volume 4 - Significant Urban Areas, Urban Centres and Localities, Section of State, July 2016 (cat. no. 1270.0.55.004).

#### Standardised death rate (SDR)

Standardised death rates enable the comparison of death rates between populations with different age structures by relating them to a standard population. The current standard population is all persons in the Australian population at 30 June 2001 (19,413,240), as published prior to recasting the ERP series. SDRs are expressed per 1,000 or 100,000 persons. There are two methods of calculating SDRs:

• The direct method - this is used when the populations under study are large and the age-specific death rates are reliable. It is the overall death rate that would have prevailed in the standard population if it had experienced at each age the death rates of the population under study; and
• The indirect method - this is used when the populations under study are small and the age-specific death rates are unreliable or not known. It is an adjustment to the crude death rate of the standard population to account for the variation between the actual number of deaths in the population under study and the number of deaths which would have occurred if the population under study had experienced the age-specific death rates of the standard population.

#### State or territory of usual residence

State or territory of usual residence refers to the state or territory of usual residence of:

• the population (estimated resident population);
• the mother (birth collection); and
• the deceased (death collection).

In the case of overseas movements, state or territory of usual residence refers to the state or territory regarded by the traveller as the one in which he/she lives or has lived. State or territory of intended residence is derived from the intended address given by overseas arrivals, and by Australian residents returning after a journey abroad. Particularly in the case of the former, this is not necessarily the state or territory in which the traveller will eventually establish a permanent residence.

#### Total fertility rate (TFR)

The sum of age-specific fertility rates (live births at each age of mother per female population of that age) divided by 1,000. It represents the number of children a female would bear during her lifetime if she experienced current age-specific fertility rates at each age of her reproductive life (ages 15 - 49).

#### Usual residence

Usual residence within Australia refers to that address at which the person has lived or intends to live for a total of six months or more in a given reference year.

## Quality declaration - summary

### Institutional environment

Estimated Resident Population (ERP) uses data sourced from a variety of institutional environments. Much of the data is administrative by-product data collected by other organisations for purposes other than estimating the population. Births and deaths statistics are extracted from registers administered by the various State and Territory Registries of Births, Deaths and Marriages. Medicare Australia client address data and Defence data is used to estimate interstate migration. Information provided by the Department of Home Affairs from their Travel and Immigration Processing System is used to calculate overseas migration.

ABS Census of Population and Housing and Post Enumeration Survey (PES) data are used to determine a base population from which ERP is calculated and to finalise all components of population change. For information on the institutional environment of the Australian Bureau of Statistics, please see ABS Institutional Environment.

### Relevance

Estimates of the resident population (ERP) is the official measure of the population of states and territories of Australia according to a usual residence population concept. ERP is used for a range of key decisions such as resource and funding distribution and apportioning seats in the House of Representatives to each state and territory. ERP for the states and territories of Australia are published by sex and age groups, and estimates and projections of the Aboriginal and Torres Strait Islander population are also available.

### Timeliness

Preliminary ERP data, disaggregated by sex and single year of age, is compiled and published quarterly and is generally made available six months after the end of each reference quarter.

Revised estimates are released once more accurate births, deaths and net overseas migration data becomes available. In the case of births and deaths, the revised data is compiled on a date of occurrence basis annually in the March reference period, released in September for the previous financial year.

In the case of overseas migration, final data is based on actual traveller behaviour. Final estimates for overseas migration are released quarterly and made available 18 months after the end of the reference period.

Final ERP estimates are made available every 5 years after a census when revisions are made to the previous intercensal period. ERP data is not changed once it has been finalised. Releasing preliminary, revised and final ERP involves a balance between timeliness and accuracy.

### Accuracy

All ERP data sources are subject to non-sampling error. Non-sampling error can arise from inaccuracies in collecting, recording and processing the data. In the case of Census and PES data every effort is made to minimise reporting error by the careful design of questionnaires, intensive training and supervision of interviewers, and efficient data processing procedures. The ABS does not have control over any non-sampling error associated with births, deaths and migration data (see institutional environment).

Another dimension of non-sampling error in ERP is the fact that the measures of components of population growth become more accurate as more time elapses after the reference period. As discussed under Timeliness, the tradeoff between timeliness and accuracy means that a user can access more accurate data by using the revised or final ERP data. While the vast majority of births and deaths are registered promptly, a small proportion of registrations are delayed for months or even years. As a result, preliminary quarterly estimates can be an underestimate of the true number of births and deaths occurring in a reference period. Revised figures for a reference period incorporate births and deaths registrations that were received after the preliminary data collection phase as well as the estimated number of registrations that have still not been received for that reference period. For more information see the Demography Working Paper 1998/2 - Quarterly birth and death estimates (cat. no. 3114.0) and Population Estimates: Concepts, Sources and Methods (cat. no. 3228.0.55.001).

After each Census the ABS uses the Census population count to update the original series of published quarterly population estimates since the previous Census. For example, 2016 Census results were used to update quarterly population estimates between the 2011 and 2016 Census. The PES is conducted soon after the Census to estimate the number of Australians not included in the Census. Adding this net undercount of people back into the population is a crucial step in arriving at the most accurate ERP possible. For more information on rebasing see the feature article Rebasing of Australia's Population Estimates using the 2016 Census in the December quarter 2016 issue of Australian Demographic Statistics (cat. no. 3101.0). At the time of rebasing to the 2011 Census, estimates for the period September 1991 to June 2006 were revised in response to a methodological improvement in the estimation of Census undercount, for more information see Feature Article: Recasting 20 Years of ERP in Australian Demographic Statistics, December Quarter 2012 (cat. no. 3101.0).

### Coherence

ERP was introduced in 1981 and backdated to 1971 as Australia's official measure of population based on place of usual residence. ERP is derived from usual residence census counts, to which is added the estimated net census undercount and Australian residents temporarily overseas at the time of the census (overseas visitors in Australia are excluded from this calculation). Before the introduction of ERP, the Australian population was based on unadjusted census counts on actual location basis. It is important to note this break in time series when comparing historical population estimates.

An improved definition for calculating overseas migration was applied from September quarter 2006 onwards. The key change is the introduction of a '12/16 month rule' for measuring a person's residency in Australia replacing the '12/12 month rule'. This change results in a break in time series and therefore it is not advised that overseas migration data calculated using the new method is compared to data previous to this. For further information see Information Paper: Improving Net Overseas Migration Estimation (cat. no. 3412.0.55.001).

The births and deaths data in this publication are not coherent with the data found in ABS births and deaths publications. This is because the revision cycle necessary to produce ERP results in a mix of preliminary births and deaths data, based on date of registration, and revised data which is a modelled estimate of births and deaths by date of occurrence. By contrast, the main tables of data in the births and deaths publications are based wholly on registration in the reference year, with some tables and analysis based wholly on date of occurrence data.

### Interpretability

ERP is generally easy to interpret as the official measure of Australia's population (by state and territory) on a place of usual residence basis. However, there are still some common misconceptions. For example, a population estimate uses the term 'estimate' in a different sense than is commonly used. Generally the word estimate is used to describe a guess, or approximation. Demographers mean that they apply the demographic balancing equation by adding births, subtracting deaths and adding the net of overseas and interstate migration to a base population. Each of the components of ERP is subject to error, but ERP itself is not in any way a guess. It is what the population would be if the components are measured well.

Population estimation is also very different to sample survey-based estimation. This is because population estimation is largely based on a full enumeration of components. In the case of the population base, only the PES used sampled data to adjust for census net undercount. In the case of the components of population growth used to carry population estimates forward, Australia has a theoretically complete measure of each component.

Another example of a common misconception relates to the fact that the population projections presented in this publication are not predictions or forecasts. They are an assessment of what would happen to Australia's population if the assumed levels of components of population change - births, deaths and migration - were to hold into the future.

### Accessibility

ERP data is available in a variety of formats on the ABS website under the 3101.0 product family. The formats available free on the web are:

• The main features which has the key figures commentary,
• Time series spreadsheets on population change, components of change and interstate arrivals and departures,