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Frequently Asked Questions: 2010-based Subnational Population Projections This product is designated as National Statistics

Released: 21 March 2012 Download PDF

Introduction

This document provides answers to frequently asked questions (FAQs) specific to the 2010-based subnational population projections.

Questions about projections in general may be answered by the general projections FAQ document, and specific methodological questions may be answered by the 2010-based SNPP methodology document.

If you cannot find the answer to your question in any of these documents please send your query to snpp@ons.gsi.gov.uk or telephone +44 (0)1329 444652.

Publication and available data

Where can I find the latest subnational projections?

The latest subnational population projections for England, the 2010-based subnational projections, are located on the ONS website.

What subnational projection data are produced/available? Can I have more projection data than is published on the website? Can I have projection data in a different format?

A set of tables containing males, females and persons, by five-year age groups, for all years, for all administrative and health geographies, including components of population change (births, deaths and migration) are produced and published as part of the release. Published figures are rounded to the nearest 100 persons.

Further breakdowns of projection data, for example by single years of age, or unrounded figures, may be available on request, and it may be possible to provide data in a format different to that presented on the website. For further information please email snpp@ons.gsi.gov.uk, or telephone +44 (0)1329 444652.

Published tables show rounded data in five-year age groups; are unrounded data available? Are single year of age data available?

It is ONS policy to publish subnational population projections presented in at least five-year age groups and rounded to at least the nearest hundred persons. The methodology used to produce the projections calculates single year of age unit-level figures, and these are available on request to enable further calculations and analysis but users should be aware that projections at this level of detail are not guaranteed to be as exact as the level of detail implied by unit-level data.

For what level(s) of geography are the subnational projections available?

Subnational projections are available for regions (formerly Government Office Regions), Strategic Health Authorities (SHAs), Primary Care Organisations (PCOs), counties and local authorities in England.

Which set of geographic boundaries were used? Are projections available on old boundaries?

The geographic boundaries used were correct as of 1 April 2009, which included the ten new unitary authorities, using the new 9-digit area codes. The SNPPs are not available on any previous boundaries, nor for any boundaries not specified in the answer above.

Are population projections for areas smaller than local authority available?

No. ONS does not produce population projections for areas smaller than local authority. Some local authorities and other analysts do produce their own projections for specific smaller geographic areas.

Why are components of change data not available for PCO geographies?

The projections are calculated on local authority boundaries, and the final population split according to proportions to make PCO-level population projections for the convenience of users. It is not possible to break the components of population change down any further without first creating small area projections, and it is not possible to simply aggregate internal migration between local authorities.

95 PCOs are coterminus with local authority boundaries, so it should be possible to use the component data for these local authorities as the component data for these PCOs. For the 34 PCOs that consist of two or more complete local authority areas, the aggregated local authority component data will also apply to the PCO area, with the exception of internal migration. The remaining PCOs are split across authorities and as such it is not possible to extract component data.

Does ONS produce variant subnational projections?

No. ONS only produces one set of principal subnational projections. Research into subnational variants is still under consideration.

Are subnational projections available for the other UK countries?

The subnational population projections published by ONS are for areas within England only. Subnational projections for other UK countries are the responsibility of the respective devolved administrations: for Scotland by National Records for Scotland (upto 2010-based), for Wales by the Statistical Directorate of the Welsh Government (upto 2008-based), and for Northern Ireland by the Northern Ireland Statistics and Research Agency (upto 2008-based).

When will the next set of subnational projections be published?

Subnational projections are usually published every two years approximately 6 months after the national projections. All planned future releases are announced on the release calendar.

Input data used

What population base was used in these projections? Did you use the indicative estimates from the latest Migration Statistics Improvement Programme (MSIP) publication?

The base year data used in these projections are the indicative 2010 mid-year population estimates published on 17 November 2011. Although they use a different estimate for the base year, the projections are consistent with the 2010-based National Population Projections for England published on 26 October 2011.

Why are the projections based on the indicative mid-2010 estimates, not the published estimates?

The indicative mid-2010 estimates are the result of the cross-government Migration Statistics Improvement Programme (MSIP) and are recognised as incorporating improvements to the method for the distribution of migration estimates to local authorities. These new data, which are only available from 2006 to 2010, have been used to ensure the projections are of the highest quality.

The published mid-year estimate series provides a consistent time series over the intercensal period from 2001 to 2010. This series will be revised when results from the 2011 Census become available.

Will the projected mid-2011 figures differ from the 2011 Mid-Year Estimates when they are available?

Yes. The 2011 Mid-Year Estimates will be based on the 2011 Census and will take account of the recorded number of births, deaths, and estimated migration in the year to mid-2011. The projections are based on estimates that have been rolled forward from the 2001 Census to 2010 and the components used are based on recent trends rather than current data that would be used in the production of the estimates.

Have data from the 2011 Census been used to calculate the subnational projections? Why were results from the 2011 Census not used in these projections?

Projections are produced with the best available information at the time. Results from the 2011 Census had not been published when the 2010-based SNPPs were calculated, so it was not possible to use any 2011 Census information. A new set of projections will be produced in due course. All planned future releases are announced on the release calendar.

What data sources are used to set the assumptions?

Registrations of births and deaths collected by the civil registration system at the General Register Office are used to calculate fertility and mortality rates. Internal migration estimates and cross-border migration estimates are based on administrative data. International migration estimates are based on data from the International Passenger Survey, together with asylum seeker data from the Home Office and National Asylum Support Service. For more detail about the methodology, please refer to the methodology document of this release.

Why is the number of births so much higher in the 2010-based SNPPs than the 2008-based SNPPs?

Broad scale changes between projections can result from changes to the national projections. While numbers of deaths and migration have increased slightly between the 2008-based and 2010-based national projections, numbers of births have increased more significantly in the short term. As a result of a national change, subnationally births must, on average, increase to match.

Why is the 2010-based projection of this area so different to the 2008-based projection?

We would expect the projections to be slightly different, but larger differences may be due to changes in:

  • the population estimate used as the base between mid-2008 and mid-2010,

  • the population estimates or components of change as a result of revisions to the estimates,

  • trend data over the trend period,

  • the national projections causing localised effects to a particular area,

  • higher national short-term fertility rates notably affecting a particular area.

If you have specific questions please send your query to snpp@ons.gsi.gov.uk, or telephone +44 (0)1329 444652.

Methodology

How are the subnational projections produced?

The SNPP methodology is based on the internationally recognised cohort component method which takes the population at the start of the year and adjusts for births, deaths and migration to calculate the population at the end of the year. For more detail about the methodology, please refer to the methodology document of this release.

What is the difference between internal, cross-border and international migration?

Internal migration covers movement of people between different local areas within England. Cross-border migration covers movement of people between the constituent countries of the UK, ie between England and Wales/Scotland/Northern Ireland. International migration covers movements of people to/from a local authority from/to outside the UK according to the United Nations definition of an international long-term migrant.

I live in an area with large numbers of Armed Forces – how are they projected?

Armed Forces personnel are treated as a ‘static population' such that they retain the same size and age-sex structure as the base year throughout the period of the projections. For more detail please refer to the methodology document of this release.

What assumptions have been made to calculate these projections?

Assumptions about numbers of births, deaths and migration are based on trend data and constrained to the 2010-based National Population Projections. For more detail about the methodology, please refer to the methodology document of this release.

Have adjustments been made to any areas? Why are adjustments made?

Occasionally the trends calculated during processing of subnational projections need to be manually altered where the trend looks implausible, or leads to implausible results. For detail about the adjustments made for this release, please refer to the methodology document.

Does ONS use local information to improve the projections?

SNPPs are projections, not forecasts, and, as such, do not attempt to predict the impact that government policies, development aims, changing economic circumstances nor other factors might have on demographic behaviour. The projections do not consider the capacity of an area to accommodate changes in its population, nor do they attempt to predict how any local-level information may affect the projections. They simply indicate the population levels and age structure that would result if the assumptions and trends based on observed estimates were to be realised. A departure from the trend-based approach is only considered where the calculated trend observed in migration is demonstrably biased due to an atypical observation.

Do the projections account for all the changes to state pension ages?

Table 4 published with the 2010-based SNPP release includes three definitions of state pension age:

  • ‘Broad age groups 1’ matching the age-group definitions used in Labour Market statistics,

  • ‘Broad age groups 2’ using the latest state pension ages as set out in the Pensions Bill 2011,

  • ‘Broad age groups 3’ using state pension ages under the provisions of the Pensions Acts of 1995 and 2007, that is, before the latest change, to be consistent with the originally published 2010-based NPPs.

How do these subnational projections relate to the national projections produced by ONS?  Are the subnational projections consistent with the national projections?

Although they use a different estimate for the base year, the 2010-based subnational projections are consistent with the 2010-based national projections for years 2011 to 2035. Across all subnational areas they sum to the national projection for England in terms of population (by single year of age and sex) and components of change (births, deaths, cross-border and international migration).

Accuracy and improvements

How accurate are the projections?

As a result of inherent uncertainty of demographic behaviour, any set of projections will inevitably be proved wrong, to a greater or lesser extent, as a forecast of future demographic events or population structure. Projections are uncertain and become increasingly so the further they are carried forward in time, particularly for smaller geographical areas. Care should be taken in interpreting these data, particularly where broken down by age and sex. The projections are more robust at greater levels of aggregation, either by age or by area, since more detailed levels mean smaller counts contributing to the projection process. Data are only published in five-year age groups at local authority level rounded to the nearest 100 persons for this reason.

Accuracy assessments, most notably the 2006-based accuracy report, have been made comparing the early years of the projected period in previous sets of projections with corresponding mid-year estimates.

How accurate are projections for small areas, or small population numbers?

Population estimates for the Isles of Scilly in particular and, to a certain extent, the City of London are considered to be less reliable given the small population in these areas. Certain individual populations (specific age groups in specific authorities) may also be very small. The projections employ the same basic methodology across the country, so these areas are treated consistently, however projections for these areas should be treated with particular caution.

What has ONS done to improve the projections?

Improvements have been made to the population estimates and some of the components of change upon which the projections are based during phase 2 of the Migration Statistics Improvement Programme. Small changes have been made to the projections processes to incorporate these improvements.

Were users/experts consulted about the subnational projections? Why was there no consultation specifically about the subnational projections?

Users were invited to comment upon the migration trends feeding into the 2010-based subnational projections as part of the MSIP consultation process. The results of this consultation were considered in the production of the 2010-based SNPPs, but no consultation was performed specifically upon the results of the SNPPs to avoid overburdening stakeholders. A response to the user feedback (45.9 Kb Pdf) is available on the ONS website.

Other

Who are the main users of the subnational population projections, and what are they used for? 

The subnational projections are used as an input into household projections, and are used by central and local government for planning education and health services provision. They are also used by local authorities for local planning or as a basis for producing their own projections, and by Communities and Local Government and the Department of Health for funding allocations to local authorities and Primary Care Organisations.

The SNPPs are used to inform local-level policy and planning. Other uses include, but are not limited to:

  • emergency service provision,

  • business development (including construction of new developments),

  • reviews of policy by officials, individuals and special interest groups,

  • survey design and development,

  • planning local events,

  • calculation of local rates, measures and indicators,

  • academic research,

  • market research.

Why is Department of Health (DH) and Communities and Local Government (CLG) using these data?

Any specific use of the subnational population projections by CLG, DH or any other organisation should be discussed with that organisation.

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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