National, state and territory population methodology

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.

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

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.

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 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.

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:

Pt+1 = Pt + B - D + NOM, where:

Pt = the estimated resident population at time point t

Pt+1 = the estimated resident population at time point t+1

B = the number of births occurring between t and t+1

D = the number of deaths occurring between t and t+1

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:

Pt+1 = Pt + B - D + NOM + NIM.

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).

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

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 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, 2009 (cat. no. 3228.0.55.001).

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'.

The number of births minus the number of deaths.

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 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 (NOM 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 (NOM departures).
     

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 or absence using the '12/16 month rule'.

NOM departures are all overseas departures that contribute to net overseas migration (NOM). It is the number of outgoing international travellers 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 stay or absence using the '12/16 month rule'.

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.

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

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.

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.

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.

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.

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 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 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.

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 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.

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.

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. 

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.

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, 1998 (cat. no. 3114.0) and Population Estimates: Concepts, Sources and Methods, 2009 (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).

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, Mar 2010 (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.

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.

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, 
• Spreadsheets containing the key data tables from the pdf version of the publication, 
• ABS.Stat datasets with population and the components of change, which can be extracted in various formats

If the information you require is not available as a standard product, then ABS Consultancy Services can help you with customised services to suit your needs. For inquiries contact the National Information and Referral Service on 1300 135 070. Alternatively, please email client.services@abs.gov.au.