1. Methodology background

 National Statistic   
 Frequency  Biennial
 How compiled  Based on third party data
 Geographic coverage  UK
 Last revised  29 October 2015

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2. Important points about national population projections data

  • National population projections (NPPs) provide statistics on potential future population levels of the UK and its constituent countries.

  • They are not forecasts and do not attempt to predict the impact of future political and economic changes.

  • They will inevitably differ to a greater or lesser extent from actual future population.

  • The projections become increasingly uncertain the further into the future they go; this means longer-term figures should be treated with caution.

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3. Overview

We publish national population projections (NPPs) every two years. We base them on the most recently available population estimates together with assumptions about future levels of fertility, mortality and migration. Each new edition of the NPPs supersedes the previous one.

NPPs are broken down by sex and single year of age and usually cover the following 100 years, although because of the uncertainty of longer-term projections we only focus on the first 25 years in our bulletins.

NPPs are used both within and outside of government as the definitive set of national population projections. Examples of their uses include informing fiscal projections, identifying future demand for health and education services and estimating the future cost of state pensions. They are also used as the base for subnational population projections and household projections.

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4. Output quality

This report provides a range of information that describes the quality of the data and details any points that should be noted when using the output.

Office for National Statistics (ONS) has developed Guidelines for Measuring Statistical Quality based upon the five European Statistical System (ESS) Quality Dimensions. This report addresses the quality dimensions and important quality characteristics, which are:

  • relevance
  • timeliness and punctuality
  • accuracy
  • coherence and comparability
  • output quality trade-offs
  • assessment of user needs and perceptions
  • accessibility and clarity

We provide more information about these quality dimensions in the following sections.

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5. About the output


(The degree to which the statistical output meets users’ needs.)

The national population projections (NPPs) serve a wide range of users across government and beyond. We produce the projections at the request of the National Statistician and the Registrars General for Scotland and Northern Ireland. As such, their content and method of production have been formally agreed and are regularly revisited to see if changes are required.

The Office for Statistics Regulation (OSR) assessed the NPPs, along with other population projections and estimates, for their compliance with the Code of Practice for Official Statistics. OSR published its findings in Report number 309 and confirmed the NPPs’ reaccreditation with National Statistics status in March 2017.

The NPP Committee, accountable to the National Statistician and Registrars General, oversees the projections process. Within the production process we and the devolved administrations consult leading stakeholders, including representatives from relevant government departments and other parties such as the Bank of England and the Local Government Association. Consultation meetings allow stakeholders to discuss proposed assumptions, highlight potential new data requirements or request changes to presentation or the publication timetable. We take into account stakeholder views and suggestions as far as possible, within the Code of Practice for Official Statistics.

We base the projections on the latest available mid-year population estimates for each UK country and also use the latest data on births, deaths, internal and international migration. However, they are not forecasts and, because of the inherent uncertainty of demographic behaviour, any set of projections will inevitably differ from actual future outcomes to a greater or lesser extent. To give users of the projections an indication of this uncertainty, we produce a number of variant population projections based on alternative assumptions about the future.

The NPPs do not attempt to predict the impact of future political and economic changes. This means, for example, that the principal projections do not make adjustments for the possible demographic consequences of the UK’s pending withdrawal from the European Union.

The projections cover a period of 100 years but uncertainty necessarily increases the further into the future they go. Long-term figures should be treated with great caution and we recommend that projections by single year of age should be treated with extreme caution when looking beyond 25 years.

We regularly consider what user needs are not being met by our published statistics – we identify these needs using the processes described in the following Assessment of user needs and perceptions section. At present known gaps in user needs include:

  • earlier publication of the NPPs: we are aware that some users would benefit from earlier publication and we will review whether this is feasible for future releases

  • more information on uncertainty around projections: our current methods do not allow the creation of measures of uncertainty; while we recognise that there is some user interest in this, further research into alternative approaches is currently a low priority

  • projections by ethnic group: for these we refer users to ETHPOP, a resource developed by demographers in academia; however, there may be scope for us to review the potential for alternative and more regular products, this is also a low priority

Timeliness and punctuality

(Timeliness refers to the lapse of time between publication and the period to which the data refer. Punctuality refers to the gap between planned and actual publication dates.)

We publish the national population projections (NPPs) every two years, usually in the October following the reference year (for example, we published the 2016-based NPPs in October 2017). This publication includes the principal projection and the main variant projections. We discuss which variants to include with our main stakeholders across government. If this process indicates demand for more variants than we are able to produce in the main release, we publish the lower priority variants around a month later.

The starting point for the projections is the population estimates for England and Wales, Scotland and Northern Ireland, which are produced by Office for National Statistics (ONS), National Records of Scotland (NRS) and the Northern Ireland Statistics and Research Agency (NISRA) respectively. The last of these estimates become available at the end of June of the year following the reference year and we publish the NPPs approximately four months later. We have always met the planned publication date.

Historically the main NPPs release was followed by a reference volume containing details of the results of the projections, the assumptions made for future fertility, mortality and migration, and also the methodology behind the calculations. However, we now publish all this information with the main release.

For more details on related releases, the ONS Release Calendar provides 12 months’ advance notice of release dates. In the unlikely event of a change to the pre-announced release schedule, we will draw public attention to the change and fully explain the reasons for the change at the same time, as set out in the Code of Practice for Official Statistics.

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6. How the output is created

We calculate the national population projections (NPPs) using a number of standard demographic methods. Full details of the methods can be found via the bulletin. Some of the main methodological points are detailed in this section.

We make the projections for successive years running from one mid-year to the next using the cohort component method, which can be summarised as:

Population (year x) plus Births (between years x and y) minus Deaths (between years x and y) plus In-Migrants (between years x and y) minus Out-Migrants (between years x and y) equals Population (year y).

For each age, the starting population plus or minus the net number of migrants (immigrants minus emigrants) less the number of deaths produces the number in the population, one year older, at the end of the year. To this we add survivors of those born during the year. We define age as completed years at the last birthday.

We use the mid-year population estimates from each country as the starting population. We calculate the numbers of births, deaths and migrants using the assumptions of future levels of fertility, mortality and migration, which we determine using a mixture of data observation and extrapolation, as well as consideration of expert opinion. We also adjust the assumptions for the first year of the projection to take into account the very latest data.

In 2012, we commissioned a migration assumptions methodology review from the University of Southampton. This made various recommendations on the methodology for setting international migration assumptions with the intention of increasing transparency and streamlining previous processes. The results are as follows.

From the 2012-based NPPs onwards we have applied overall international migration assumptions, rather than breaking them down by world area.

Also from the 2012-based NPPs onwards we have produced separate international migration assumptions for each UK country.

For the 2012-based NPPs we used ARIMA models (a standard approach to time series extrapolation) to determine the principal international migration assumptions. However, the rise in net migration by 2014 led to the models projecting values that were deemed improbably high. In consequence, for the 2014-based NPPs we used a 25-year rolling average of international migration to and from the UK. For the 2016-based NPPs we modified this to a straight 25-year average covering the period 1992 to 2016, so avoiding the weighting towards the latter end of the series. Our Expert Advisory Panel and delegates at the NPP consultation supported the use of this approach. For the 2012-based NPPs we also used ARIMA models for the cross-border flows between the four nations of the UK. However, since the 2014-based NPPs we have calculated the cross-border flows using rates of movement by age and sex rather than fixed numbers – this avoids the projections producing implausible values, such as negative population stocks, when projected fixed levels of out-migration are bigger than the initial population size.

The 2001 National Statistics Quality Review of methodology for projecting mortality assessed various projection methods and concluded that no alternative method would be likely to outperform the existing method. It recommended some amendments to the methodology such as setting a target year at 25 years hence and using UK rather than England and Wales data to analyse past trends. These changes were implemented in subsequent projections. In January 2015, we initiated a new review into the methodology for setting mortality assumptions. We implemented some enhancements for the 2014-based NPPs, including a new method of smoothing historical mortality data to analyse past trends and determine the rates of mortality improvement for 2014, the initial year of the projections.

Following the 2014-based projections we commissioned the University of Southampton (UoS) to review the methodology for producing the mortality assumptions. UoS developed a new methodology for modelling mortality improvement rates using a Generalised Additive Model with separate models for infant and old age mortality. We have carried out initial testing on the model and will carry out further testing and development over the next two years with a view to transitioning to the model for the 2018-based projections.

We have used a consistent method for the fertility assumptions for several rounds of projections. We assume a completed family size (CFS) for the 25th year of the projections and then set the trajectory between the current level of fertility and the long-term CFS using analysis of recent trends, an assessment of their implications for future CFS and expert opinion.

We compute the population for each of the constituent countries of the UK and add the results together to produce projections for England and Wales (combined), Great Britain and the UK. We produce variant projections using the same method but with alternative assumptions of future levels of fertility, mortality and migration. We have published details of these alongside the release. Previously we calculated some variant projections on a non-additive basis, meaning we calculated the projections for each geographic level separately. This meant that the totals for England and Wales (combined), Great Britain and the UK didn’t necessarily match the sum of the constituent country totals.

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7. Validation and quality assurance


(The degree of closeness between an estimate and the true value.)

The national population projections (NPPs) use the latest official population estimates as their base year and are inevitably dependent on the accuracy of these estimates. The accuracy of these estimates can be assessed after a census has been carried out. We calculate the population estimates using the internationally recognised cohort component method – starting with the population data from the last decennial census and updating each year with the available data on births, deaths and migration. The report Population estimates for local authorities across UK constituent countries: a comparison of data sources and methods compares the population estimates methodology used across the four UK countries.

We consider each component of the projections – fertility, migration and mortality – separately when setting the assumptions for each set of projections. The assumptions are based largely on extrapolation of past trends. Inevitably there is some element of subjective judgement and choices of main assumptions are also informed by the views of an Expert Advisory Panel, as well as by NPP consultation events where we meet our main stakeholders. We provide more detail on the respective Expert Advisory Panel with each publication.

Because of the inherent uncertainty around future demographic behaviour we often hold long-term assumptions of each component constant. International migration numbers are particularly unpredictable, with recent figures varying widely, so we usually assume a constant level from just a few years into the projection period – for example, in the 2016-based projections we assumed annual net migration of 165,000 per year, applying from the year ending mid-2023 onwards. Fertility measures are less volatile, although as a period measure the total fertility rate (TFR) is affected by when women choose to have their children as well as the number that they choose to have. We usually assume the TFR to be static about 10 to 15 years into the projection. We measure mortality in terms of improvements in mortality rates and, as changes are more gradual and stable, assume the current rates gradually converge to a standard rate of improvement at the same level, for all but the oldest ages, 25 years into the projection. We review all these time periods for each new set of projections.

It would be improbable for any projection to correspond entirely with the actual demographic outcome. Changes in government policy, in the economy, in individual, family and household behaviour and in events outside the UK will influence the three main components of population change. Our variant projections offer a set of plausible alternative scenarios according to higher or lower assumptions about the trajectories of fertility, migration and mortality.

Some of the variants combine alternative assumptions – for example, a “young” population assumption (high fertility, high migration and low life expectancy assumptions). Other variants allow users to decompose the projections to increase understanding of how changes in the assumptions affect the projected population. For example, by comparing the zero net migration variant with the principal projection, the impact of the level of assumed migration in the principal projection can be assessed. The variant projection results are also available in the release.

The method we use to produce the projections does not enable statements of probability to be attached to them, or for confidence intervals to be ascribed to the variant projections. Therefore, the levels of uncertainty for the fertility, mortality and migration assumptions are not directly comparable. However, study of past data shows that international migration has been more volatile than the other components.

It is also possible to look at earlier projection sets and compare them with what subsequently happened. In July 2015, we published an accuracy report, which compared NPPs with population and migration estimates and births and deaths data up to 2013, where available. In 2007, we published the article Fifty years of United Kingdom population projections: how accurate have they been?. A previous analysis covering the projections made during the period 1971 to 1991 is in Population Trends number 77 – details of this are in the Index to Articles 1975 to 2003.

All of these articles consider how close to the actual outcome the NPPs have been, the errors for each of the three individual assumptions and whether accuracy has improved in more recent projections. They also discuss the variant projections. These analyses are inevitably dependent on comparisons with the latest population estimates. Revisions to estimates of the past and current population (for example, the revisions made to population estimates following the 2001 and 2011 Censuses) also play a part in explaining projection error. Revisions may make the projections look more or less accurate than they really are.

The 2007 article UK national population projections in perspective: How successful compared to those in other European countries by Nico Keilman (University of Oslo) offers an international comparison of accuracy.

Quality assurance of administrative data

In January 2015, the Office for Statistics Regulation (OSR) issued the Regulatory Standard for the quality assurance of administrative data. This new standard applies to all official statistics where administrative data are used in the production of these statistics. All producers of official statistics that use administrative data need to implement this requirement, by embedding good practice into their production to assure the quality of the data.

In response, the Office for National Statistics (ONS) Population Statistics Division has published a set of Quality Assurance of Administrative Data reports for all of the administrative datasets that underlie its products. Many of these underlie the national population projections either directly or indirectly (via the population and migration estimates).

Coherence and comparability

(Coherence is the degree to which data that are derived from different sources or methods but refer to the same topic, are similar. Comparability is the degree to which data can be compared over time and domain, for example, geographic level.)

Each set of national population projections (NPPs) is unique, comprising assumptions made using the best information available at a point in time. Thus each new set of projections, using the most up-to-date background data available, supersedes the previous set. Although the results of subsequent projections can be compared, this will not be comparing like-with-like but instead observing what effect the most recent data, when built into projections for the future, will have on the expected future population of the country.

All NPP publications from the 2014-based onwards are available on our current website. The complete set of online materials up to the 2014-based NPPs is available on the National Archives website. This includes the NPP historic series page containing data from the 1954-based to 2004-based projections.

The only known official population projections for the UK apart from those produced by Office for National Statistics (ONS) are those produced by Eurostat and the United Nations (UN). Eurostat’s EU population projections refer to population as at 1 January, rather than at mid-year as for the NPPs. Each time Eurostat creates a projection, ONS supplies a 1 January population estimate by single year of age and sex to use as the base population. Responsibility for the production of these base estimates falls within the Population Estimates Unit of ONS. Eurostat has no fixed timetable for their population projections, but their stated aim is to produce a projection set every three to four years.

Eurostat also use the cohort component approach to calculate their projections but use their own methods to decide assumptions for fertility, mortality and migration. In consequence their projections differ from ours.

The UN produces worldwide population projections every two years. They publish a combination of population estimates and projections. The UN uses a Bayesian method to project the fertility and mortality assumptions and then the standard cohort-component model to project the population forward. Individual countries such as the UK have no input into the UN population projections.

Concepts and definitions

(Concepts and definitions describe the legislation governing the output and a description of the classifications used in the output.)

Conceptual framework: a conceptual framework for population and migration statistics (including the population estimates) is available.

Usually resident: population projections and the estimates they are based on, include the “usually resident” population only. This is the standard UN definition and includes only people who reside in a country for 12 months or more. As such, short-term international migrants are excluded.

Components of change: population changes between one year and the next are caused by three components of change: births (fertility), deaths (mortality) and migration. To inform the projections we make assumptions about how each of these will change in future.

Fertility: in a demographic or projections context fertility relates to how many children a group of women have, rather than their ability to conceive (which is the common understanding of fertility).

Mortality: the likelihood of death, often presented as mortality rates – the proportion of a group of a particular age and sex who die during the course of the year.

Mid-year: 30 June of any given year, where the period from one mid-year to the next is from 1 July of year x until 30 June of year x plus one.

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8. Other information

Output quality trade-offs

(Trade-offs are the extent to which different dimensions of quality are balanced against each other.)

We publish projections up to 100 years to serve known user requirements. However, projections become increasingly uncertain the further they are carried forward, so the long-term figures should be treated with great caution.

Assessment of user needs and perceptions

(The processes for finding out about uses and users and their views on the statistical products.)

Because we publish national population projections (NPPs) for each of the four UK nations, we work closely with our partners in the devolved administrations, including discussing appropriate methods. We do this via our twice-yearly NPP Committee meetings, as well as ongoing liaison.

We collect other information on users’ needs for, and perceptions of, the NPPs in a number of ways:

  • meetings with other primary stakeholders, such as government departments and the Bank of England – this includes the NPPs consultation events and also bespoke events occurring just after publication
  • user groups, for example, the Central and Local Information Partnership and the Population Theme Advisory Board Group – allowing our main users to comment on existing plans and to put forward changes in their requirements
  • contact with individual users – drawing on the evidence provided by users who contact us with requests for, or queries on, the projections
  • web analytics data showing use made of specific parts of a release
  • feedback on NPPs may also come from Office for National Statistics (ONS) engagement with users, including its annual customer satisfaction surveys

The information collected through these methods feeds into decisions on content and formats of outputs.

In general, feedback on the NPPs themselves and the way we support our customers is positive – for example, the key account assessment in the 2015 to 2016 ONS customer survey shows that multiple departments praised our engagement. The “Relevance” section previously notes the main user requirements that we are not currently fulfilling.

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9. Sources for further information or advice

Accessibility and clarity

(Accessibility is the ease with which users are able to access the data, also reflecting the format in which the data are available and the availability of supporting information. Clarity refers to the quality and sufficiency of the release details, illustrations and accompanying advice.)

The national population projections (NPPs) are provided as HTML web pages, including graphs and interactive tools to aid interpretation. We also make textual background information and supporting documents, including methodological information, available as part of each release.

The data files are provided in Microsoft Excel format and also, from the 2014-based NPPs onwards, in XML format. The XML files contain more detailed data than have previously been released to provide users with the data they need for their own modelling work. Users may also download the bulletin in PDF format.

For information regarding conditions of access to data, please refer to the links:

Any feedback on or further enquiries about the NPPs should be directed to pop.info@ons.gov.uk or, by phone, to +44(0)1329 444661.

Useful links

2016-based national population projections

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Contact details for this Methodology

Andrew Nash
Telephone: +44 (0) 1329 44 4661