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Methodology: Interim 2011-based subnational population projections This product is designated as National Statistics

Released: 28 September 2012 Download PDF

Introduction

This report documents the methodology used in producing the interim 2011-based subnational population projections for England published on 28 September 2012.

Subnational projections are usually published every two years by the Office for National Statistics, most recently the 2010-based projections on 21 March 2012. These interim projections have been produced ahead of the usual schedule of releases to meet specific user requirements for projections based on data from the 2011 Census, before all the data usually required to update trends in the projection model are available.

Subsequent sections give more detailed information about the actual data and methods used, however key features of these interim projections include:

  • These projections are based on the 2011 mid-year population estimates published on 25 September 2012 which are rolled forward from the 2011 Census,

  • Assumptions about future fertility, mortality and migration levels are based on the trends calculated for the 2010-based subnational population projections. A revised back series for the rolled forward mid-2002 to mid-2010 population estimates taking account of the 2011 Census is not yet available to update the assumptions,

  • They have been projected ten years ahead to 2021 and published for each English region (formerly Government Office Region), county and local authority (LA) by single year of age and sex.

Projections are not forecasts and take no account of policy nor development aims that have not yet had an impact on observed trends. The interim 2011-based subnational population projections for England provide an indication of the possible size and structure of the future population, based on the 2011 mid-year population estimate and the continuation of estimated trends used in the 2010-based projections.

The 2011 population estimates for England are 452,000 higher than the previously published 2010-based projection for 2011. This has an impact on the starting point of the projection and the age structure, which can also affect fertility, mortality rates and internal migration. Assumed levels of future migration are not affected. The statistical bulletin provides further discussion of these impacts and additional user guidance. This will be of interest to users, especially those who use the projections for modelling, planning and research purposes. 

Overview of methodology

The subnational population projections use the internationally accepted cohort component methodology. These interim projections take the local authority 2011 mid-year population estimates as their starting point and assume the continuation of estimated trends in fertility, mortality and migration used in the 2010-based projections. To model trends, data for up to six preceding years are used, so for the 2010-based projections this meant data from years 2005 to 2010 were used. Users should note that the 2010 projections were based on the indicative 2010 mid-year population estimates published 17 November 2011 and not the published series of mid-year estimates. The population has been projected forward ten years from the base year to 2021 for each local authority, by age and sex.

The projections for each year are calculated by first removing the population of armed forces  who are treated as a ‘static population’ whose size and age-sex structure does not change over the projection period. The population from the previous year is then aged-on, local fertility and mortality rates are applied to calculate projected numbers of births and deaths, and the population is adjusted for internal, cross-border, and international migration. For example, for the first year of these interim set of projections, the mid-2011-12 change is applied to the mid-2011 base to produce the new mid-2012 population estimate.

Each component (except internal migration) is constrained to its respective total in a national population projection for England (NPP) produced as described in the England control figures section. Similarly, once the static population has been added back, the projected population is controlled to these England control figures. This process is repeated for each year of the projection period.

The diagram below illustrates the projection process.


Base population

The 2011 mid-year population estimates, rolled forward from the 2011 Census and published on 25 September provide the starting point as the base data for these projections. These estimates refer to the usually resident population at their usual place of residence. This includes all those temporarily away from home (for six months or less) and excludes visitors. Armed forces stationed outside England are not included, but those stationed inside England are included. Asylum seekers and visitor switchers (people who enter a country intending to visit, but stay 12 months or more to become usual residents) now residing in England are included. Students are taken to be resident at their term-time address.

The resident population is divided into two types for the purposes of projection:

  • the civilian population,

  • armed forces (both home and foreign).

The civilian population refers to the usually resident population excluding home and foreign armed forces who are also usually resident. Home and foreign armed forces constitute two separate population types and are treated as separate static populations in that their size and age and sex structures are assumed to remain constant over the projection period. Data for 2011 on UK and foreign armed forces are from the 2011 Census. They include numbers of home and foreign armed forces living in barracks and army quarters. They do not include armed forces dependants; these are included in the civilian population.

Resident armed forces are removed from the usually resident population to create the civilian population at the start of processing each projection year. The civilian population is then aged-on one year to become the appropriate age in the following year of the projection. For example 17 year-olds in Birmingham in one year will become the basis for the 18 year-olds in Birmingham for the next year. The population is then adjusted for births, deaths and migration, and in the final stage the resident armed forces are added back in.

Births

Birth data come from registered births collected by the General Register Office by local authority, age of mother (ages 15 to 44 inclusive) and sex of child.

Projected numbers of births are calculated by applying assumed local authority age-specific fertility rates (ASFRs) and the national sex ratios to derive a number of births, by age of mother and sex of child. These are then constrained to add up to the number of births projected in national population projections for England produced as described in England Control Figures section, and finally these are added to the aged-on population.   

Detailed methodology

As stated in the introduction, these interim 2011-based projections use the trends calculated for the 2010-based subnational population projections. In the 2010-based projections, local authority level ASFRs were created for each of the past five years (2006-2010) using births between mid-year points by age of mother and the population at the end of that period. National level ASFRs are calculated in a similar way using the total number of births in a year and the total population at the end of the year. The sum of the five local ASFRs is then divided by the sum of the five national ASFRs to create an average differential for each local authority. The differentials are then applied to the national ASFRs from the first year of the national population projections in order to calculate local ASFRs. This process is repeated using a rolling five-year average to produce local ASFRs for each year of the projection period.

The projected number of births is then calculated for each year by multiplying the local level ASFRs by the number of women of the corresponding age, local authority and year. Projected total births are then split by sex of child using a fixed national sex ratio at birth.

The total number of births is then controlled to the national projected total of births by dividing the national projected birth data by the aggregated local authority birth data for each combination of age of mother against sex of child. This gives a scaling factor for each age/sex combination, which are then applied to the local authority level data. This method ensures that the component, in this case births, sums to the national total. As stated in the introduction, applying fertility trends from the 2010-based projections to the new 2011 base impacts the number of births projected (see the statistical bulletin for further discussion of these impacts).

Sometimes it is necessary to adjust fertility rates in an area due to small populations producing volatile rates. The following adjustments made in the 2010-based subnational projections have also been applied to this interim 2011-based set of projections, however no further adjustments were made:

  • Fertility rates were capped to be no greater than five times the national fertility rate. It is not known how many area were affected by this capping,

  • For the Isles of Scilly, fertility rates were replaced by the assumed fertility rated used in the national population projections for England. This is because fertility rates for this area are highly volatile due to its small population size.

These births then become the bases for the infants of the year being projected.

Deaths

Death data come from registered deaths collected by the General Register Office by local authority, age and sex.

Projected numbers of deaths are calculated by applying assumed local authority level age/sex-specific mortality rates (ASMRs) to derive numbers of deaths, by age and sex. These are then controlled to add up to the number of deaths projected in the national population projections for England, and finally these are subtracted from the aged-on population.

Detailed methodology

Local authority level ASMRs are calculated in a similar method to that used for the fertility rates. ASMRs are created for each of the past five years using number of deaths occurring between mid-year points by age and sex and the population at the end of the year. As these interim projections assume a continuation of 2010-based mortality trends this means data for 2006-2010 was used. National level ASMRs are calculated in a similar way using the total number of deaths in a year and the total population at the end of the year. The sum of the five local ASMRs is then divided by the sum of the five national ASMRs to create an average differential for each local authority. The differentials are then applied to the national ASMRs from the first year of the national population projections in order to calculate local ASMRs.  This process is repeated using a rolling five-year average to produce local ASMRs for each year of the projection period.

The projected number of deaths is then calculated for each year by multiplying the local level ASMRs by the population for each age and sex in each local authority.

The total number of deaths is then controlled to the national projected total of deaths by dividing the national projected death data by the aggregated local authority death data. This gives scaling factors by age and sex which are applied to the local authority level data. This method ensures that the number of deaths sums to the national total. See the statistical bulletin for user guidance on how updating the base to 2011 and the resulting changes in the age structure impacts the number of deaths projected.

The following adjustment made in the 2010-based subnational projections has also been applied to the interim 2011-based to improve the projections of deaths in local areas:

  • Mortality rates were capped to be no greater than five times the national mortality rate. It is not known how many areas were affected by this capping.

No further adjustments were made.

These deaths are then subtracted from the aged-on population.

Internal migration

Adjusting for the expected number of people entering and leaving a local authority by age and sex is done separately for internal, cross-border and international migration using different methodologies. This section describes the data sources and methods for internal migration.

An internal migrant is defined as someone who changes their local authority of residence between one year and the next. In the subnational population projections, internal migration is defined as migration between areas within England only. For some other uses internal migration is defined as including migrant flows between England and Wales, Scotland and Northern Ireland, but in the subnational projections these are referred to as cross-border flows. This is because the methodology used to project internal migration requires a full matrix of flows into and out of each local authority by single year of age and sex. This level of detail is not required to project cross-border migration and it is therefore treated separately.

Migration is recognised as the most difficult component of population change to estimate as there is no compulsory system within the UK to record movements of the population. At present ONS uses a combination of three administrative data sources as a proxy for internal migration within England and Wales: the National Health Service Central Register (NHSCR), the Patient Register Data Service (PRDS) and Higher Education Statistics Agency (HESA) data. ONS use these data sources to calculate the internal migration component of the mid-year population estimates, which forms the basis upon which projected internal migration is calculated. Further information is available in the internal migration estimates methodology guide (125.5 Kb Pdf) . The internal migration estimate data used to calculate trends in the 2010-based subnational population projections were produced using the same methods as used in the published population estimates series. The only difference being due to a correction of a processing error in estimating moves of students between local authorities at the start and end of their studies.

Internal migration estimates produced by ONS provide an origin-destination matrix which provides information on moves from each local authority to every other local authority by sex and single year of age for each of the five trend years (2006-10). To project internal migration moves, five-year trend data is used to estimate the proportion of the population that has left a particular local authority and where they have moved to. By adding up the estimated number of outflows of internal migrants from every other authority into a particular authority, the inflows into that authority are calculated. The following detailed methodology section provides more information on the impact that updating the base to 2011 has on internal out-migration and the subsequent effect on internal in-migration.

Detailed methodology

1. The proportion of people moving out from any one local authority (LA) is calculated by dividing the number of people moving out of that LA by the number of people living there. This is calculated separately for males and females by single year of age for each of the trend years individually and then an average is calculated, using five years trend data, to produce rates of out-migration. 

The out-migration rates are applied to the aged-on civilian population (after adjusting for births and deaths) in each authority in order to estimate the number of internal out-migrants from each LA for the projected year.

2. In some local authorities with small numbers of moves and/or populations, this can lead to atypical rates which produce unrealistic results in the projected population. To overcome this, adjustments are sometimes made to smooth the data. These can take the form of upper limits (or caps) on migration rates, or the replacement of rates with appropriate alternatives. The following adjustments applied in the 2010-based projections have also been applied to the interim 2011-based: 

  • The assumed proportion of people of any individual age-sex group moving out of an area is capped at a maximum rate of 0.8.

  • The small population size of the Isles of Scilly results in unrealistic propensities to migrate. They were replaced with those of Cornwall which were scaled (doubled) to produce out-migration trends that were more realistic.

  • Rutland and Oadby and Wigston have very small populations for some age-sex groups, in these areas adjustments were made to the migration propensities of a specific subset of age-sex groups. For Rutland this involved replacing the proportions migrating by age for all the females and using the male propensities by age in their place. In Oadby and Wigston the male propensities to migrate for those aged 24 to 30 was replaced by the female proportions for the same age.

The interim 2011-based projections apply propensities to migrate from the 2010-based historic trend data for the years 2005 to 2010. However where the differences between the size and age distribution between the mid-2011 base and the 2011 estimate from the 2010-based projections are large, internal out-migration numbers have been affected. 

Quality assurance indicated that in some areas, mainly for age groups where migration rates are higher, the resulting projected internal out migration was unrealistic compared to recent estimates of trends. Therefore additional adjustments have been made to the migration propensities for selected ages in a number of areas. Areas selected had at least two five year age groups (across the ages 19 – 60) where the difference between the 2011 mid-year estimate and the 2010-based projection for mid-2011 was more than 33 per cent. Adjustments are made by multiplying the internal out migration propensity by a scaling factor for a subset of ages. The scaling factors were decided by comparing the original propensity to migrate with a proxy rate calculated using the 2011 mid-year estimate. Adjustments made are detailed in the table one.

Table 1: Areas, sex, ages and scaling factors used to adjust internal migration propensities in the 2011-based interim subnational population projections

Area Sex and ages Scaling factor applied
Bournemouth Males 26 to 33 0.7
Bournemouth Females 25 to 32 0.7
Cambridge Males 22 to 24 0.8
Cambridge Males 25 to 37 0.75
Cambridge Females 22 to 24 0.9
Cambridge Females 25 to 35 0.65
Malvern Hills Females 19 to 26 0.65
North Dorset Males 20 to 27 0.45
North Dorset Females 20 to 27 0.6
Richmondshire Males 19 to 33 0.7
Rutland Males 20 to 27 0.5
Rutland Females 20 to 27 0.5
Tunbridge Wells Females 19 to 27 0.65
West Dorset Males 20 to 26 0.65
West Dorset Females 20 to 26 0.65

Table source: Office for National Statistics

Download table

3. To distribute the projected out-migrants to a destination local authority, the origin-destination matrix (by age and sex) is used. The probability of a person moving to local authority B given that they are moving from local authority A is calculated by dividing the number of people moving from A to B by the total number moving out of A using the five years’ trend data.

4. The total inflow for each authority is calculated by adding the outflows from every other authority into this particular authority.

5. Net internal migration for each local authority, by age and sex, is calculated by subtracting outflows from inflows. The total net internal migration across all local authorities in England must sum to zero because these are movements within the country not between countries.

The population is then adjusted for these internal moves between areas and at the end of this step the population has been adjusted for internal migration. 

Cross-border migration

Adjusting for the expected number of people entering and leaving a local authority by age and sex is done separately for internal, cross-border and international migration using different methodologies. This section describes the data sources and methods for cross-border migration.

Cross-border migration between England and the rest of the UK is captured in a similar way to internal migration flows. Flows between England and Wales are produced using the same data sources as for internal migration.

Information on moves in to, and out of, Scotland and Northern Ireland are collected and treated differently from moves within England and Wales, by using data from National Records of Scotland and the Northern Ireland Statistics Research Agency. Further information is available in the internal migration estimates methodology guide (125.5 Kb Pdf) .

To calculate cross-border moves, an average of five years’ historic data (2006-10) from the 2010-based projections has been used to give an average count of moves between local authorities in England and the other countries of the UK (Wales, Scotland and Northern Ireland). The assumption is that this average remains constant for the whole projection period; however, as with the other components, cross-border migration is controlled to the national population projections, by age and sex for each year, so according to the national projected cross-border moves the local authority level figures may be scaled up or down.

The population is then adjusted for these cross-border moves and at the end of this step we have a temporary population which has been adjusted for cross-border migration.

International migration

Adjusting for the expected number of people entering and leaving a local authority by age and sex is done separately for internal, cross-border and international migration using different methodologies. This section describes the data sources and methods for international migration.

The NPP international migration assumptions are made in terms of total net migration. This is split down into a number of streams (international migration, visitor switchers, and asylum seekers) for use in the subnational population projections with the inflows (immigration) and outflows (emigration) modelled separately and the data sources and methods are described for each stream below.

Once the assumed inflows and outflows for each international migration stream have been calculated, the outflows from each stream are subtracted from their respective inflows to calculate the net flow for each stream by local authority, age and sex. These net flows are then added to the temporary population, which was created in the preceding migration step.

International migration to and from the rest of the world (including the Republic of Ireland but excluding the UK)

The main source of information on international migration is the International Passenger Survey (IPS). This is a voluntary sample survey of passengers travelling through airports, seaports and the Channel tunnel. It provides information on the number of people intending to stay in, or leave, the UK for 12 months or more.

For immigration (international inflows) an average of five years’ historic trend data (2006-2010) from the 2010-based projections has been used to give an average count of moves of international migrants into local authorities in England. The assumption is that this average remains constant for the whole projection period; however, as with the other components, this inflow is controlled to the national population projections, by age and sex for each year, so according to the national projected immigration the local authority level figures may be scaled up or down.

The method used for calculating emigration (international outflows) in the interim 2011-based subnational population projections remains unchanged from the 2010-based subnational projections, although an improvement was made to the standardisation method used which has resulted in different projected emigration to the 2010-based projections, although the historic trend data used are the same. This component requires six years of historic trend data, (the same data as used in the 2010-based subnational projections) as the emigration component of the population estimates is modelled using a three-year average. To ensure that each year’s trend data is not more heavily weighted than any other, six years of trend data are averaged using weights based on data at a national and regional level to create an average count of moves of international migrants out of local authorities. Within this process the assumption is that this average remains constant for the whole projection period; however, as with the other components, this outflow is controlled to the national population projections, by age and sex for each year, so according to the national projected emigration the local authority level figures may be scaled up or down.

Visitor switchers

Visitor switchers are people who visit, or leave, the UK intending to stay for less than 12 months, but who actually stay for 12 months or longer, thus becoming migrants. These switchers are identified by the IPS as they complete their journey when subsequently entering or leaving the UK (the passenger is asked how long they intended to stay in the UK or overseas when they initially arrived or departed, and how long they actually remained in or out of the UK).

Visitor switcher numbers are based on the assumptions from the 2010-based projections. That is, 2010 data are used to give the number of visitor switchers at local authority level which is then constrained to the national projections to produce the assumptions. The subnational projections assume that visitor switcher levels remain constant throughout the projection period.

Asylum seekers

Data on asylum seekers and their dependants are provided by the Home Office and the National Asylum Support Service. Applications for asylum (excluding an estimate of those removed from the UK within one year and a small number of asylum seekers captured by the IPS) provide the basis for estimated inflows of asylum seekers. Data on removals, refusals, withdrawals and appeals for principal applicants and dependants are used to estimate outflows of asylum seekers leaving the UK after 12 months or more without being captured by the IPS.

As with visitor switchers assumptions from the 2010-based subnational population projections have been used. Again the numbers are assumed to remain constant throughout the projection period and Asylum seeker data at local authority level are controlled to the NPPs to produce the assumptions. 

Final constraining stage

At the England level, the subnational population projections are consistent with the corresponding set of national population projections produced according to the method described in the England Control Figures section.            

After adjusting for natural change and migration, the home and foreign armed force populations are added back to the adjusted civilian population, and a final constraining stage is undertaken.

The birth, death and migration components have all been controlled to the corresponding England data at the end of each projection year. However, the subnational projections components of change do not always fully explain the change in total population between one year and the next. This is due to a difference in the processing order and way mortality and fertility rates are applied in the national population projections and the subnational population projections. Consequently a final controlling step takes place to ensure that the subnational population projections add up to the national population projections by both age and sex. This is done as the last process in the cycle of producing the projection for a year which then forms the base population for the next year’s calculation. This process is repeated to produce each year’s subnational population projections.

England control figures

These interim 2011-based projections have been controlled to a set of 2011-based population projections for England up to 2021. These controls have been produced using the 2011 mid-year population estimate, published on 25 September and the principal fertility (47 Kb Excel sheet) , mortality (197 Kb Excel sheet) and migration (60 Kb Excel sheet) assumptions from the 2010-based National Population Projections published 26 October 2011, as set for 2011 onwards. Further information on how the 2010-based NPP assumptions were set is in the 2010-based Background and Methodology Report on the ONS website.

Outputs

These interim 2011-based projections have been produced ten years from the base year to mid-2021. They are published unrounded for each English region (formerly Government Office Region) and local authority (LA) by single year of age and sex.

Projections become increasingly uncertain the further they are carried forward due to the inherent uncertainty of demographic behaviour, and at lower levels of disaggregation by age and area. Therefore users are advised that they should aggregate the projections to five year age groups and round to the nearest hundred.

These projections are not forecasts and do not attempt to predict the impact that future government or local policies, changing economic circumstances or other factors might have on demographic behaviour.

Further information about the Subnational Population Projections may be obtained from the Office for National Statistics, Population Statistics Division, Population Projections Unit, Segensworth Road, Titchfield, Fareham, Hampshire, PO15 5RR.
Telephone:  +44 (0) 1329 444652
Email: projections@ons.gsi.gov.uk

Background notes

  1. Details of the policy governing the release of new data are available by visiting www.statisticsauthority.gov.uk/assessment/code-of-practice/index.html or from the Media Relations Office email: media.relations@ons.gsi.gov.uk

    These National Statistics are produced to high professional standards and released according to the arrangements approved by the UK Statistics Authority.

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