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Chapter 1: Background and Methodology, 2010-based NPP Reference Volume This product is designated as National Statistics

Released: 29 March 2012 Download PDF

Introduction

The 2010-based national population projections for the UK and its constituent countries were produced by the Office for National Statistics (ONS) on behalf of the National Statistician and the Registrars General of Scotland and Northern Ireland. This set of projections is based on a review of the underlying assumptions regarding fertility, mortality and migration and was agreed in liaison with the statistical offices of the devolved administrations – Welsh Government, National Records of Scotland (NRS) and Northern Ireland Statistics and Research Agency (NISRA) – following consultation with key users of projections in each country and advice from an expert advisory panel.

Purpose and history

Purpose

The primary purpose of the national projections is to provide an estimate of the future population of the UK (and of its constituent countries) which is used as a common framework for national planning in a number of different fields. A new set of projections are normally produced every two years based on assumptions which are judged to be most appropriate from the statistical evidence available at the time. These official sets of projections ensure that the many users of projections can work on consistent assumptions. However, due to the 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. To give users of the projections an indication of this uncertainty, a number of variant population projections have also been produced based on alternative assumptions of future levels of fertility, mortality and migration and are discussed in the variants chapter.

Principal and key variant projections were published by ONS on 26 October 2011, and additional variant projections were published on 23 November 2011. Full results of the principal and variant projections are available on the ONS website.1

Projections History

The first projections of the population of the UK were made by the Government Actuary’s Department (GAD) in the 1920s.  One of the main uses of these earliest projections was in connection with long-term financial estimates under the Contributory Pensions Acts and other schemes of social insurance. In 1954, responsibility for the production of the official national projections was given to the Government Actuary and since then they have been increasingly used in all areas of government planning.  New projections were made each year from 1955 to 1979 and then every second year until 1991. There was then a 1992-based set and since then projections have been produced every second year. Additional ‘interim’ 2001-based and 2003-based projections were carried out following, respectively, the 2001 Census and subsequent revisions to population estimates for England and Wales.

Responsibility for the production of national population projections transferred to ONS on 31 January 2006. Full results of all previous sets of projections are available on the GAD website.

Publications

This volume, the latest in a regular series started in 1970, gives full details of the 2010-based national population projections produced by ONS (based on the estimated population at the middle of 2010). These replace the previous 2008-based projections published on 21 October 2009.

Expert advisory panel

As part of the production process for the 2004-based projections round an expert academic panel was created and met in 2005 to advise on the assumptions underlying the national projections. This panel met again in 2007 to advise on appropriate assumptions for the 2006-based projections, in 2009 to advise on the 2008-based projections and again in 2011 for the 2010 based projections. The role of this expert panel is strictly advisory; responsibility for final decisions on the assumptions remains with ONS and the statistical offices of the devolved administrations. A note of the 2011 meeting of the expert panel is included in the 2010-based projections October release, Appendix A.

Base population and projection period

The projections are based on estimates published on 30 June 2011 by ONS of the resident population of the UK at mid-2010 3 of some 62.3 million people. This estimate is based upon 2001 Census results with allowance for subsequent births, deaths, migration and ageing of the population. The population includes all usually resident persons, whatever their nationality. Members of HM armed forces in the UK are included, but members of HM armed forces and their families who are abroad are excluded and are treated as migrants when they return home. Foreign armed forces stationed in the UK are included.

Base populations for individual countries

The projections for England and Wales are based on the mid-2010 population estimates published by ONS on 30 June 2011. The projections for Scotland are based on the mid-2010 population estimates published by NRS on 27 April 2011, and likewise the projections for Northern Ireland are based on the mid-2010 estimates published by NISRA on 30 June 2011. Table 1.1 shows the estimates of the population at mid-2010 upon which these projections are based.

Table 1.1: Base population estimates for 2010-based projections

thousands
England 52,234
Wales 3,006
Scotland 5,222
Northern Ireland 1,799
United Kingdom 62,262

Table source: Office for National Statistics

Download table

Estimates of the population aged 90 and over

Official mid-year population estimates produced by ONS, NRS and NISRA are prepared by individual age to age 89, with an upper age band for all those aged 90 and over. Estimates of the population aged 90 to 109 and 110 and over are prepared using the Kannisto Thatcher survivor ratio method,4 with the results controlled to agree with the official estimates of all those aged 90 and over.

Estimates for those aged 90 and over 5 are now published for England and Wales on an annual basis.  Scotland have published population estimates of people aged 90 to 104,6 by single year of age, and the number of people aged 105 and over, for 2002 to 2010.

Projection period

Data have been published up to 100 years into the future. This reflects the increasing interest in long-term issues such as pensions and health care. However, the main focus of these projections is on the first 25 years to 2035. Longer-term results are discussed in this volume where appropriate, however, the long-term figures should be treated with great caution since population projections become increasingly uncertain the further they are carried forward.

Methodology

The cohort component method

The projections are made for successive years running from one mid-year to the next using the cohort component method.7  For each age, the starting population plus net inward migrants less the number of deaths produces the number in the population, aged one year older, at the end of the year. To this has to be added survivors of those born during the year.  Age is defined as completed years at the last birthday.

Migration is assumed to occur evenly throughout the year. For computing purposes, this is equivalent to assuming that half the migrants in a given year at a given age migrate at the beginning of the year and half at the end of the year. The number of net migrants to be added to obtain the population aged x+1 at the end of the projection year therefore consists of half of those migrating during the year at age x and half of those migrating during the year at age x+1.

The number of deaths in a year is obtained by adding half of the net inward migrants at each age to the number in the population at the beginning of the year, then applying mortality rate qx (known as the initial mortality rate, or the probability of dying).

The number of births in a year is calculated by multiplying the average number of women at each single year of age during the year (taken as the mean of the populations at that age at the beginning and end of the year) by the fertility rate applicable to them during that year. The total number of births in a year is assumed to be divided between the sexes in the ratio of 105 males to 100 females, in line with recent experience.

The number of infants aged 0 at the end of the year is calculated by applying a special ‘infant mortality rate’ to the projected number of births, and adding half the number of net migrants aged 0 last birthday. This special mortality rate is equivalent to about 85 per cent of the conventional full first year of life infant mortality rate used in official statistics.

The projections are computed for each of the constituent countries of the UK and the results are added together to produce projections for England and Wales, Great Britain and the UK.

Lexis diagram

The projection process can be illustrated by means of a Lexis diagram (see Figure 1.1) where age is represented on the vertical axis and time on the horizontal axis. The life of an individual (or of a birth cohort), is represented by a diagonal line (or parallelogram) running from bottom left to top right.

The line AB represents the population aged x at mid-year y. The size of this cohort one year ahead, that is, aged x+1 at mid-year y+1, is represented by the line DC. To calculate this population one year ahead (for x greater than or equal to 0), it is necessary to project deaths and net migration occurring to this cohort between mid-year y and mid-year y+1. The relevant interval of time for this cohort is represented by the shaded parallelogram ABCD.

The net number of migrants aged x between mid-year y and mid-year y+1 is represented by the square ABDE in the Lexis diagram. Similarly, the net number of migrants aged x+1 between mid-year y and mid-year y+1 is represented by the square BFCD.

As noted above, it can be assumed that half the migrants in a given year at a given age migrate at the beginning of the year and half at the end of the year. Thus, of net migrants aged x between mid-year y and mid-year y+1, it can be assumed that half add to the population represented by the line AB in the diagram and the other half to the population represented by the line ED. Similarly, of the migrants aged x+1 in this period, half can be added to the population denoted by the line BF and half to the population represented by the line DC. Net migration in the parallelogram ABCD is therefore obtained by adding half of the net migrants aged x (that is, those adding to the population AB) and half of those aged x+1 (that is, those adding to the population DC) in this interval.

Figure 1.1

The number of deaths in a year is obtained by adding half of the net inward migrants at each age to the number in the population at the beginning of the year and applying the mortality rate qx. This produces the number of deaths in the parallelogram ABCD. Finally, there is the special case of projecting the number of infants aged 0 at mid-year y+1 as described above. So if x=0, the required population is represented by the line ED and it is therefore necessary to project births, deaths and net migration in the triangle represented by ADE.

The relationship between mx and q x

The mortality rates (qx) are given, for each individual age, in the data tables available on the ONS website. However, in other statistical publications, and in the mortality chapter, mortality rates are often shown as central death rates (mx). These are obtained by dividing the number of deaths during a year at a given age by the average population at that age during the year (usually taken to be the population at the midpoint of the year). The relationship between qx and mx is shown by the following equation:

Note that this equation is an approximation as it assumes deaths occur evenly between exact age x and exact age x+1. It does not hold for infant mortality, as infant deaths are concentrated in the first few months of life.

The qx rates used in the projections are the results of two interpolations. The first interpolation takes place between the qx rates for adjacent calendar years and produces rates on a mid-year to mid-year basis.  The second interpolation is between adjacent ages and gives a set of qx rates that, in life table terms, relate to exact age x+½ on a mid-year basis.  These are assumed to be applicable to the mid-year population at age last birthday.

 

Data availability

Website

Detailed results of the 2010-based national population projections for the UK and its constituent countries are available from the ONS website.1 The results include the principal and variant projections for each country, and a summary of the assumptions upon which they are based. The key datasets can be downloaded in Microsoft Excel format for each country. There is also a NPP interactive table download tool.15

For each projection, the following datasets can be downloaded:

  • components of change, summary age distributions and dependency ratios

  •  population in five-year age groups

  • population by individual age (to age 90)

  •  fertility rates by individual age

  •  fertility rates in five-year age groups

  •   mortality rates by individual age

  •   net migration by individual age

The projected population numbers are shown in thousands but stored to three decimal places (that is, to unit level). This does not imply that the projections are accurate to that level of detail. Results should always be presented in thousands.

Additionally, period and cohort life expectancy data 17 derived from historic mortality rates (from 1981 to 2010) and assumed calendar year mortality rates from the 2010-based national population projections are also available.

Responsibility for the production of the National Population Projections transferred to ONS in 2006. However, there is a complete historic projections database, which includes detailed projection data from the 1954-based to 2006-based projections, available on the GAD website.

Further information

In addition to the standard projection output listed above, the following further details are available on request from ONS:

  • births by individual age of mother

  • deaths by individual age

  • population at individual ages over 90

Further information about the National Population Projections may be obtained from the Office for National Statistics, Population and Demography Division, Population Projections Unit, Segensworth Road, Titchfield, Fareham, Hampshire, PO15 5RR.

Telephone:  +44 (0) 1329 444652

E-mail: projections@ons.gsi.gov.uk

References

1.  Full results of the 2010-based principal and variant projections can be found at: www.ons.gov.uk/ons/publications/all-releases.html?definition=tcm%3A77-21600

2.  Responsibility for the production of national population projections transferred to ONS on 31 January 2006. Full results of all previous sets of projections are available at: www.gad.gov.uk/Demography%20Data/index.html

3.  Mid-2010 estimates for each constituent country of the UK are available at: www.ons.gov.uk/ons/publications/all-releases.html?definition=tcm%3A77-22371

4.  Thatcher AR, Kannisto V and Andreev K. The Survivor Method for Estimating Numbers at High Ages. Demographic Research – Max Planck Institute online journal, Vol 6-1 (2002). Available at: www.demographic-research.org/volumes/vol6/default.htm

5.  Mid-2010 estimates of the very elderly (including centenarians). Available at: 
www.ons.gov.uk/ons/rel/mortality-ageing/population-estimates-of-the-very-elderly/2010/index.html

6.  Population Estimates for Scottish Centenarians. Available at:
www.gro-scotland.gov.uk/statistics/theme/population/estimates/special-populations/centenarians/index.html

7.  For a good introduction to projections methodology see Chapters 16 to 18 of Hinde A Demographic Methods.  Arnold. (1998)

8.  Subnational Population Projections (England). Available at: www.ons.gov.uk/ons/publications/all-releases.html?definition=tcm%3A77-21850

9.  2010-based Population Projections for Scottish areas. Available at:
www.gro-scotland.gov.uk/statistics/theme/population/projections/sub-national/index.html

10.  2008-based marital status projections for England & Wales. Available at: 
www.ons.gov.uk/ons/publications/all-releases.html?definition=tcm%3A77-23636

11.  Communities and Local Government (2010). Household projections to 2033, England. Available at: www.communities.gov.uk/publications/corporate/statistics/2033household1110

12.  See: http://budgetresponsibility.independent.gov.uk/pubs/Economy-Supplementary-Tables-AS11.xls

13.  Regional population projections EUROPOP2010. Available at: http://epp.eurostat.ec.europa.eu/portal/page/portal/population/publications/population_projections

14.  World Population Prospects: Available at: http://esa.un.org/wpp/unpp/panel_population.htm

15.  National population projections interactive data download tool can be found at: www.ons.gov.uk/ons/external-links/other-government-departments/neighbourhood-statistics-download-tables.html

16.  UK interactive population pyramid: www.ons.gov.uk/ons/guide-method/understanding-ons-statistics/interactive-content/dvc3-twin-national-populations-pyramid-content-item.html

17.  Period and cohort life expectancy data: www.ons.gov.uk/ons/rel/lifetables/period-and-cohort-life-expectancy-tables/2010-based/index.html 

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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