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An Overview of Population and Migration Statistics

Related links
Population Estimates for UK, England and Wales, Scotland and Northern Ireland
Short Term Migration Estimates for England and Wales
Migration Statistics Quarterly Report
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Population statistics describe the size, make up and change of the UK population.

These include statistics on: the number, age and sex structure, and geographic distribution of the population; the factors driving population change (births, deaths and migration); and on topics such as families and older people.

This overview focuses on statistics published by the Office for National Statistics (ONS).

Where these statistics cover only part of the UK, similar statistics for other parts are often published by the relevant statistical agency - namely National Records of Scotland; the Welsh Government; and the Northern Ireland Statistics and Research Agency.

Population and migration statistics have a wide range of uses; they are used by central and local government and the health sector for planning and monitoring policy and service delivery; resource allocation; and managing the economy. Additionally, they are used by a wider range of organisations such as commercial companies for market research, special interest groups and academia as well as being of interest to the general public. The data are also used to meet international obligations under European Union law, and to provide information to international statistical bodies such as Eurostat, the OECD, United Nations, and ILO.

This page describes the aspects of population and migration ONS measures and why. Information on how these are measured, and the statistics themselves, can be found via the links provided.

How the UK population is affected by inflows, births and immigration, and outflows, deaths and emigration

Population Size

ONS estimates annually the size of the “usually resident population” at the mid-year point (30 June). This is the population who have been usually resident for a period of at least 12 months, or, if they have been resident for less than this period, intend to stay for at least 12 months in total.  Visitors and short term immigrants are not included, but usual residents of the UK temporarily visiting another country are included.

Births, deaths, and long-term international migration will affect the size of the usual resident population.  At the local level the usual resident population is also affected by internal migration.

ONS produces estimates for the UK, the constituent countries, and for Local Authorities.  ONS also produces estimates for other geographies such as parliamentary constituencies, national parks, wards, and health areas.

For any given area, breakdowns by age-sex will generally be estimated.  In addition, breakdowns will be provided by other characteristics when the data allow and there is a user need.

In the UK, the population is estimated using the cohort component method, which estimates the population by taking the previous year’s population estimates and adjusting for births, deaths and migration (plus other minor adjustments, for example accounting for armed forces). A census is undertaken every ten years to create a new baseline for population estimates.

Estimating the size of the very old population between censuses is particularly difficult. Separate estimates are published that split the estimates of the very old population into additional age bands, using an alternative method.

The Census

The census, which has been conducted every ten years since 1801 (apart from 1941) provides detailed estimates and a baseline for the population as it was on Census Day. From 1981, the Census has measured the usually resident population.  In addition the 2011 Census has also published data on alternative definitions to usual residence, including the workday population and the short-term resident population.

These estimates can be broken down by characteristics - for example, country of birth or marital status, and can be provided for small areas.

The census aims to collect information from every member of the population.  This means that the census has the advantage of providing sufficient numbers of smaller groups of the population for analysis - such as minority ethnic groups.

Census estimates have the disadvantage of becoming outdated as the decade passes but can provide many statistics not currently available from other sources.

ONS has established the Beyond 2011 programme to investigate and develop alternatives to the traditional census.

Population projections provide a picture of the population as it may develop in future years.

Projecting the Population into the Future

ONS publishes both national projections, of the usually resident population, for the UK and its constituent countries, and subnational projections for local authorities and higher areas in England, every two years.

Population projections are based on mid-year population estimates and a set of underlying assumptions regarding future fertility, mortality and migration. They are not forecasts and do not attempt to predict the impact that current and future government policies, changing economic circumstances or other factors might have on demographic behaviour, for example, government policies on immigration, student fees, or local house-building policies.

In addition to estimates and projections of the numbers of households in England are published by the Department for Communities and Local Government.

Measuring Migration

Migration of usual residents

While population estimates and projections measure the population for a specific area at a point in time, migration statistics generally relate to the flow of people between one location and another over a period of time.

Measuring those who change their place of usual residence provides a direct link to the definitions used for population estimates and projections.  This allows estimates of long term international migration and of migration between areas within the UK to be directly applied as components of change in the calculation of population estimates and projections.

Long term international migration statistics (0.99 Mb Excel sheet) provide estimates of the number of usual residents moving in or out of the UK. This set of statistics is probably the one most quoted when discussing international migration. They include only those intending to change their place of usual residence for 12 months or more, thus long-term immigrants are added to and emigrants are subtracted from the estimate of usual residents of the UK . The difference between long-term immigration and emigration is net migration change.

Internal migration estimates by Local Authority measure the flow of usual residents around the UK by age and sex.  They are published annually and are used as components of change in Local Authority population estimates and projections.

Migration data on other definitions

Estimates of long term international and internal migration, based on usual residence status, are required for estimating or projecting the population; other measures and definitions are needed to gain a full insight into migration.

Short term migration statistics provide estimates of the number of people who enter or leave the country for at least one or three months (depending on definition) and less than 12 months.  It is not considered that these individuals have changed their place of usual residence so these statistics are not used in the calculation of ONS usual residence based population estimates or projections.

Information on visitors is published monthly and quarterly in overseas travel and tourism statistics.  These statistics include short term migrants, and cover all trips to and from the UK of less than a year, including those classed as short term migrations.

Long term migration, short term migration, and visitor data together provide a fuller picture of those entering the UK, covering all lengths of stay.

The Annual Population Survey (APS), Labour Force Survey (LFS) and decennial census ask for “usual address one-year ago”.  Where this address is outside the UK it provides an alternative measure of immigration, which can be combined with other information from these sources.  However, this question alone provides a snapshot with no indication of how long the person is staying in the UK.

The measures described so far concern flows; the number of people present at a given time who have ever migrated to the UK is also of interest.  The Census collected information on those who have previously migrated to the UK, including when they arrived. Estimates of the total number of people by country of birth and nationality living in the UK are also produced from the Census and APS. The LFS is used to produce estimates of employment and unemployment by nationality and country of birth.  These estimates of sub-populations (not flows) give a longer term indication of the impact of immigration and how it changes the characteristics of the population overall.

The Migration Statistics Quarterly Report brings together a variety of statistics from across government to help build up a picture of migration.  It is part of a suite of statistics published quarterly on immigration, including more detailed information published by the Home Office and Department of Work and Pensions.

The Home Office publishes data relating to the operation of the immigration control system, including: the number and type of visas issued; data on applications for British citizenship or permission to stay in the UK; and data on asylum seekers.  The legislative context and government policy can have a significant impact on migration of nationals from outside the European Economic Area (EEA), and much of these Home Office data exclude nationals of countries in the EEA who are subject to a lesser level of immigration control.

The Department for Work and Pensions publishes data on the nationality of those allocated new National Insurance numbers.

Measuring Life Events

Life event statistics contribute to the understanding of the size and condition of the population.  The number of births and deaths affect the size of the usual resident population.  Definitive data on the number of births and deaths are published on a calendar year basis, whereas data for updating population estimates use mid-year to mid-year (1 July to 30 June) data, with some other minor differences.  Statistics relating to major life events are produced including:

Exploring Demographics of the UK Population

Demographic analysis brings together a range of statistics to provide a better understanding of the country's population.

  • Fertility statistics - Fertility projections based on evidence from research are used in the population projections. Fertility analysis also adds value to information published on births reporting including statistics on childlessness, birth order and fertility among women born outside the UK

  • Life expectancy statistics combine population estimates and statistics on the number of deaths to derive an estimate of how long, on average, people of various ages can expect to live.  Related mortality projections are used in the population projections

  • Ageing analysis builds on life expectancy and population estimates, summarising the latest analysis of official statistics on population ageing and the lives of older people, and providing ageing indicators such as median age, and sex ratios at older ages, at the national and local level

  • Families analysis adds value to the information published on marriages, divorces and civil partnership, including analysis of children living in lone parent families, the number of married versus cohabiting couples, and young adults living with their parents 

Content from the Office for National Statistics.
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