Skip to content

Methodology: 2010-based Subnational Population Projections This product is designated as National Statistics

Released: 21 March 2012 Download PDF

Introduction

This report provides details about the methods and data used in producing the 2010-based subnational population projections for England published 21 March 2012.

The 2010-based subnational population projections for England provide an indication of the possible size and structure of the future population, based on the continuation of recent demographic trends, for English regions (formerly Government Office Regions), local authorities (LA), Strategic Health Authorities and Primary Care Organisations (PCOs). They are produced on a consistent basis across all local authorities in England. These projections are based on the indicative 2010 mid-year population estimates published 17 November 2011 and not the published series of mid-year estimates.  They are also consistent with the 2010-based principal national population projections for England published 26 October 2011.

The projections are trend-based, making assumptions about future fertility, mortality and migration levels based on trends in recent estimates, usually over a five-year reference period. They give an indication of what the future population size and age-sex structure might be if recent trends continued. They are not forecasts and take no account of policy nor development aims that have not yet had an impact on observed trends.

The Department for Communities and Local Government (CLG) use projections as an input into producing household projections, and as an input into funding allocation models for local authorities. The projections are also used by the Department for Health for healthcare planning, by central and local government for modelling and planning purposes, and by various other groups for planning and research purposes.

The subnational population projections usually have a consultation period on the migration assumptions that feed into the projections. For the 2010-based projections users were invited to comment on trends that would affect the subnational population projections as part of the consultation on the indicative 2010 mid-year population estimates published on 17 November 2011. The results of this consultation were considered in the production of the 2010-based subnational population projections, but no consultation was performed specifically, to avoid overburdening stakeholders.

Overview of methodology

The subnational population projections use the internationally accepted cohort component methodology. They take the local authority mid-year population estimates as their starting point and assume that recent demographic trends continue. To model recent trends, data for up to six preceding years are used, so for example in the 2010-based projections, trends are based on data from years 2005 to 2010. These project forward 25 years from the base year, the ‘projection period’, 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.

Each of these components (except internal migration) is constrained to its respective total from the corresponding principal national population projections, and once the static population has been added back, the projected population is controlled to the national population projections (NPP) total for England. This process is repeated for each year of the projection period.

The diagram illustrates the projection process.

The population at the end of each cycle becomes the base population of the next cycle.

Projections for Primary Care Organisations are not produced directly using this method but are based on the projections created for local authorities. In most cases PCO areas are coterminous with local authorities or aggregations of local authorities, in which case projections for these areas are calculated by aggregating the appropriate local authority projections. Where areas are not coterminous, PCO projections are produced by apportioning local authority level projections, based on estimates of PCO populations, by age and sex, for the base year of the projections.

Base population

The indicative 2010 mid-year population estimates published on 17 November 2011 provide the starting point as the base data for these projections, not the published series. 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 on UK armed forces are supplied by the Defence Analytical Services Agency and data on foreign armed forces originate mainly from US Air Force statistics. 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 2010 will become the basis for the 18 year-olds in Birmingham for 2011. The population is then adjusted according to the rest of the projection process, 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 controlled to add up to the number of births projected in the national population projections for England, and finally these are added to the aged-on population.

Detailed methodology

Local authority level ASFRs are created for each of the past five years 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 NPP 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.

The following adjustments were made in the 2010-based subnational projections to improve the projections of births in local areas:

  • Fertility rates were capped to be no greater than five times the national fertility rate. It is not known how many areas were affected by this capping,
  • For the Isles of Scilly, fertility rates were replaced by the assumed fertility rates 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 infants for 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. 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 NPP death data by the 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.

The following adjustment was made in the 2010-based subnational projections to improve the projections of deaths in local areas:

  • Mortality rates were capped to be no greater than 5 times the national mortality rate. It is not known how many areas were affected by this capping.

 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 the Office for National Statistics (ONS) uses a combination of three administrative data sources as a proxy for internal migration within England: the National Health Service Central Register, the Patient Register Data Service and Higher Education Statistics Agency  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 on the methodology used to estimate internal migration is available. The internal migration estimate data used to calculate trends in the 2010-based subnational population projections has been 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. 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 number of outflows of internal migrants from every other authority into a particular authority, the inflows into that authority are calculated.

Detailed methodology

  1. The proportion of people moving from one local authority is calculated by dividing the number of people moving out of an 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 a five-year average is calculated to produce rates of out-migration by age and sex. 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:

    • 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. For the 2010-based subnational population projections 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.
  2. 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 for the projected year.

  3. To distribute the projected out-migrants to a destination local authority, the origin-destination matrix is used. The probability of a person moving from local authority A 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 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. The net internal migration adjustment for each local authority, by age and sex, is calculated by subtracting outflows from inflows. The total net internal migration adjustment across all local authorities in England must sum to zero, as 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 we have a temporary population which 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 as internal migration flows. Flows between England and Wales are produced using the same data sources as the internal migration.

Information on moves in to, or 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.

To calculate cross-border moves, an average of five years’ trend data is 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, so according to the national projected cross-border moves by age and sex for each year, 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. The data sources and methods are described for each stream below.

Of the five main components of population change used to calculate the subnational projections, the methodology and data used to estimate international migration have changed the most, due to improvements resulting from the Migration Statistics Improvement Programme. This has affected both immigration and emigration. However, the same basic method of using recent trends is still applied for producing projections.

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 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.

Up to mid-2008, estimates of migration from the Republic of Ireland were made separately from IPS flows and were therefore treated separately in the subnational population projections methodology. In the 2010-based projections, migration to and from the Republic of Ireland has been included in IPS flow data and is therefore projected along with other international migration flows.

One other change has been made to the international migration methodology used in the 2010-based subnational projections. These projections are based on the migration estimates from the indicative mid-year estimates which used new methods to distribute in-migrants to local authorities. These were based on distributing migration estimates directly from the national to local authority level using administrative data sources. Details about this new methodology were published with the indicative mid-year population estimates. The immigration estimates that fed into the 2008-based subnational projections were calculated using a modelling method.

For immigration (international in-flows) an average of five years’ trend data is 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 in-flow is controlled to the national population projections, so according to the national projected immigration by age and sex for each year, the local authority level figures may be scaled up or down.

The 2010-based subnational population projections use the emigration component from the indicative mid-year estimates to measure trends in emigration. Although the modelling method for producing the emigration component for the estimates has not changed since the 2008-based projections were produced, the immigration estimate is used as an input to the model. Therefore the emigration estimates are indirectly affected by the changes in the methods used for producing local authority immigration estimates. Details about the how these emigration estimates were produced was published alongside improvements to the mid-2008 population estimates published in November 2009.

The method used for calculating emigration (international out-flows) in the 2010-based subnational population projections remains unchanged from the 2008-based subnational projections. This component requires six years of trend data 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. The assumption is that this average remains constant for the whole projection period; however, as with the other components, this out-flow is controlled to the national population projections, so according to the national projected emigration by age and sex for each year, 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 long-term 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).

The subnational projections assume that visitor switcher levels remain constant throughout the projection period. The most recent year of data is used to give the number of visitor switchers at local authority level, which is then controlled to the NPPs to produce the assumptions.

Asylum seekers 

Data on asylum seekers and their dependants are provided by the Home Office and the National Asylum Support Service. Applications for asylum provide the basis for estimated inflows of asylum seekers, these exclude an estimate of those removed from the UK within one year and a small number of asylum seekers captured by the IPS. 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.

As with visitor switchers, only the most recent year of data is used, and again the numbers are assumed to remain constant through the projection period. Asylum seeker data at local authority level are controlled to the NPPs.

Final constraining stage

At the England level, the subnational population projections are consistent with the corresponding national population projections produced by ONS. The underlying assumptions used in the national projections are agreed in liaison with the devolved administrations following consultation with key stakeholders and after seeking expert advice. 2010-based national population projections data and supporting documentation are available on the ONS website.

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 NPP 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 methodology used in the national population projections and the subnational population projections in the processing order and way mortality and fertility rates are applied. 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

The 2010-based national population projections are based on the published mid-year estimates series and therefore in the first year of the projection a greater level of constraining is required. The base population used for the 2010-based subnational projections is 20,600 smaller than the base of the 2010-based national population projections, this means that tables showing the components of change for the first year of the projection period are less likely to sum to the population than later years.

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.

Get all the tables for this publication in the data section of this publication .
Content from the Office for National Statistics.
© Crown Copyright applies unless otherwise stated.