1. What data do the Office for National Statistics (ONS) produce?

An overview of our key international migration statistics and their data sources can be found in the first time user guide. In addition to these regular publications, you can find more articles relating to international migration, covering topics such as the labour market, housing, ethnicity and the number of British migrants living abroad on the ONS website.

All of our data tables are listed in the international migration table of contents.

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2. Questions relating to students

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3. Is it possible to produce alternative breakdowns of net migration, for example excluding students or asylum seekers?

Office for National Statistics (ONS) may consider alternative breakdowns of any published figures where it would be helpful. Current sources that measure net migration limit the types of alternative variations that can be produced, for example, the International Passenger Survey (IPS) is not suitable for identifying how many students leave the UK in isolation since some non-EU students may remain in the UK after their studies and switch to non-study visas, apply for settlement and others may eventually become British citizens. However, administrative data sources are being developed that may provide a better understanding of student migration in the UK, particularly what students do following their studies. Development plans for improving international migration statistics are outlined in an article published February 2017: International migration data and analysis: Improving the evidence and presented at a Public Policy Forum on migration statistics in September 2017.

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4. Can we use International Passenger Survey (IPS) formal study data to estimate the net balance of students entering and leaving the country?

Care should be taken when comparing inflow and outflow by main reason for migration. Returning migrants are asked their reason for returning, not their original reason for migrating. A former immigrant's main reason for leaving the UK may well differ from their previous main reason for immigrating into the UK. Because of this and to avoid potential confusion about the contribution that particular groups of migrants make to total net migration figures, the previously published “balance” figures by reason have been removed from Tables 2 and 3 in the accompanying datasets.

For example, a migrant who arrived in the UK to study may subsequently leave to take up a job abroad and so not be included in an estimate of net migration for formal study.

In 2012, a new question was introduced to the IPS to collect information on an emigrant's main reason for previously immigrating to the UK. This categorised emigrants by whether they had previously immigrated for work, study, to accompany or join, or other reasons. The Office for Statistics Regulation (OSR) carried out a review of The quality of the long-term student migration statistics, which was published on 27 July 2017. The main focus of their report was the “student migration gap” – the difference between the estimate of the number of migrants entering the UK for formal study (student immigration) and the estimate of the number of former students leaving the UK (former student emigration). OSR noted that the IPS emigration estimate of former students does not provide a complete and coherent picture of former student emigration, as these figures alone do not provide information on all the different outcomes for international students. Given this, a requirement from the OSR was to label the former student emigration estimate as Experimental Statistics. What’s happening with international student migration? provides the latest update on our work in this area.

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5. Why have the former student emigration estimates been labeled as experimental?

Recently, a range of concerns about the robustness of the estimate of former student emigration were raised with the Office for Statistics Regulation (OSR). As a result, OSR carried out a review of the quality of the long-term student migration statistics, which was published on 27 July 2017. OSR investigated a number of factors that determine the extent to which the IPS accurately captures long-term student migration, including the sample design, sources of bias, and the precision of the estimates. The main focus of their report was the “student migration gap” – the difference between the estimate of the number of migrants entering the UK for formal study (student immigration) and the estimate of the number of former students leaving the UK (former student emigration).

The estimate of former student emigration is the only source of information about when a student leaves the UK; the other sources of information do not confirm the point at which the student has left the country. This lack of ability to verify and triangulate the estimate means that assurances cannot be given to provide the same level of confidence in the former-student emigration figures. OSR noted they were: “concerned that the former student emigration estimate does not bear the weight that is put on it in public debate. This estimate should add clarity on the pattern of student migration in the UK. Instead, it creates doubts by not providing a complete and coherent picture of former-student emigration, as these figures alone do not provide information on all the different outcomes for international students”.

OSR reported that: “It is standard practice for new figures to be labelled as experimental while they bed in and it is unfortunate that this was not followed in this case when the new breakdown of emigration figures by previous reason for immigration was first introduced”.

OSR therefore asked Office for National Statistics (ONS) to make clearer that this estimate should be treated with caution and that it be labelled as an experimental component of the overall National Statistics on migration, while the ONS work programme continues. This judgement applies only to the student migration component of ONS’s migration statistics; it is not a judgement about the quality of the overall estimates of immigration and emigration derived from the International Passenger Survey (IPS).

As a result, from 24 August 2017 we labelled the former student emigration estimate as Experimental Statistics in all publications. Once we are satisfied that we have a sufficient understanding of former student outcomes, including the extent to which the IPS accurately captures student departures, we can request OSR reassess the former student emigration estimate. Our latest understanding of student migration can be found in "What's happening with international student migration?".

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6. Questions relating to the European Union

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7. What impact has the EU referendum had on levels of international migration?

The first full year of data since the EU referendum vote, published in November 2017, shows a decrease in the number of people coming to live in the UK and an increase in the number leaving, resulting in a fall in net migration. Over three-quarters of the fall in net migration was accounted for by EU citizens. However, the decline follows historically high levels of immigration. These changes suggest that Brexit is likely to be a factor in people's decision to move to or from the UK – but decisions to migrate are complex and other factors are also going to be influencing the figures. The Long-Term International Migration (LTIM) and International Passenger Survey (IPS) estimates cannot directly attribute changes in long-term international migration to multiple factors.

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8. What impact did EU accession had on levels of international migration?

In 2004, there were 10 additional countries – the EU8 (Czech Republic, Estonia, Hungary, Latvia, Lithuania, Poland, Slovakia and Slovenia) as well as Malta and Cyprus – that acceded to the EU. Net EU migration increased in the years immediately following accession, especially from EU8 countries.

In 2007, Romania and Bulgaria (EU2) acceded to the EU. Between 2007 and 2013, these countries were subject to transitional controls restricting their access to the UK labour market. These restrictions were lifted on 1 January 2014. Following these restrictions being lifted, net migration doubled from 21,000 in 2013 to 44,000 in 2014.

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9. Questions relating to comparison of data sources

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10. Why do flows and stocks data not match? Can we compare the two?

Flows and stocks measure two different things:

  • flows measure the number of people moving into and out of the UK

  • stocks the number of non-UK born or non-British citizens resident in the UK at a given point in time.

The stock of the migrant population is built up over a long time period from migration flows and so the two measures are not directly comparable. This is discussed further in the Note on the differences between Long-Term International Migration flows derived from the International Passenger Survey and estimates of the population obtained from the Annual Population Survey.

Data on flows and stocks may appear to display different trend. The migration inflows may suggest a downward trends while the stock of people living in the UK who were born outside the UK has gone up. This can be explained by a situation as follows.

Assume a starting point of 0 people from a particular country of birth in the UK. If 100,000 such people enter the UK in the first year (and all stay) but in the next year the inflow halves so only 50,000 enter (and all stay), the total stock would have risen to 150,000.

In other words, stocks of people born outside the UK can continue to rise despite reduced inflows, and only go down if the combined totals of emigration and deaths exceed immigration.

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11. Is it possible to sum together the Long-Term International Migration (LTIM) data and Population of the UK by country of birth and nationality data?

No, it is not possible to sum together these estimates as the two surveys they are based on have definitional differences. LTIM estimates from the International Passenger Survey (IPS) measure the flow of migrants into and out of the country in a given time period, whereas the Annual Population Survey (APS) produces a count of the number of migrants (stock) currently living in the UK.

Further information on the differences between the Labour Force Survey (LFS) and APS, and IPS and LTIM can be found in the report published 1 December 2016: Note on the differences between Long-Term International Migration flows derived from the International Passenger Survey and estimates of the population obtained from the Annual Population Survey.

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12. Why do IPS formal study estimates not match student visas figures?

The International Passenger Survey (IPS) formal study figures provide an estimate of the number of migrants indicating their main reason for migrating is to attend a formal study course in the UK. This may not match other measures of "student" immigration such as the number of student visas issued, for a number of reasons, such as:

  • IPS estimates only include those people who intend to stay for 12 months or more, whereas visas data include people coming to the UK for less than 12 months to study short courses

  • the IPS category includes accompanied children, who are not included in the student visas figures

  • the IPS only records the stated main reason for entering the UK (this may not correspond with the nature of the visa they hold because they may also have other reasons for immigrating)

For further details please see Comparing sources of international migration statistics.

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13. Why do estimates of long-term international migration differ from the number of National Insurance numbers allocated?

On 12 May 2016, we published an information note explaining the reasons why long-term international immigration figures from the International Passenger Survey (IPS) could differ from the number of National Insurance number (NINo) registrations to adult overseas nationals.

It noted that the two series are likely to differ due to:

  • short-term migrants not being accounted for in IPS long-term international migration figures, but short-term migrants coming to the UK to work for example, for a summer job) can be allocated a NINo; therefore, the two sources are accounting for different groups of people

  • timing – overseas nationals can apply for a NINo as soon as they arrive in the UK, or at any point during their stay, therefore, a migrant may be counted by IPS as they enter the country in the latest quarter, but not by NINo registrations; this is because NINo registrations do not necessarily reflect a recent move to the UK

For further details please see Comparing sources of international migration statistics and more detail on the NINo and IPS comparison.

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14. Have results from the 2011 Census been compared with long-term international migration estimates?

Every 10 years the census provides the opportunity to compare mid-year population estimates with a count of the population at a given point in time. Population estimates are produced from administrative records on births and deaths and Long-Term International Migration (LTIM) estimates, in addition to other adjustments.

The census-based 2011 mid-year population estimate for England and Wales was 464,000 or 0.8% higher than the mid-year population estimates rolled-forward from the 2001 Census base. There are several possible causes for this small difference but it was considered that the “largest single cause is most likely to be underestimation of long-term immigration from central and eastern Europe in the middle part of the decade”. This was before improvements were made in 2009 to the International Passenger Survey (IPS).

In light of these differences, Office for National Statistics (ONS) published a revised series of net migration estimates for 2001 to 2011 as part of a review into the Quality of Long-Term International Migration estimates from 2001 to 2011. The main findings of the review were:

  • there is evidence that shows the IPS missed a substantial amount of immigration of EU8 citizens that occurred between 2004 and 2008, prior to IPS improvements from 2009

  • the IPS has underestimated the migration of children

  • the IPS improvements have both reduced the relative error around the IPS estimates, as well as improving the balance of the sample

  • there is no evidence to suggest that the current methodology used in LTIM calculations needs adjusting

The adjustments applied increase the overall estimate of net migration across the decade, but most particularly in 2005 to 2008, when the evidence suggests that the majority of migrants who were missed by the IPS immigrated to the UK.

In 2009, improvements were made to the IPS to make it much better focused on migration and to increase the geographical coverage of ports of entry to the UK. For more information see International Passenger Survey: Quality Information in Relation to Migration Flows. It is important to note that if these improvements had been made prior to 2009, then ONS would have expected the rolled-forward population estimates and the 2011 Census count to have been closer.

Users who wish to see a more detailed breakdown of inflows and outflows of long-term international migrants between 2001 and 2011 by variables such as reason for migration, age and sex, citizenship and country of birth should continue to use the existing LTIM and IPS annual tables, but should bear in mind the caveat that the headline net migration estimates have now been revised.

Further plans for continuous improvement of migration estimates include the following:

  • addressing ongoing issues with the quality of migration estimates for particular subgroups of the population

  • exploring whether additional administrative data sources can be used to improve the quality of migration and population estimates

  • continuous quality assurance of the IPS estimates

Future development plans for improving our data sources are outlined in more detail in International migration data and analysis: Improving the evidence.

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15. Where can I request further information on international migration?

It is possible to request more detailed information and data on international migration by emailing the Migration Statistics Unit at migstatsunit@ons.gsi.gov.uk.

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16. Migration terms and definitions

Annual Population Survey (APS)

The Annual Population Survey (APS) is a continuous household survey, covering the UK, with the aim of providing estimates between censuses of main social and labour market variables at a local area level. The APS is not a stand-alone survey, but uses data combined from two waves from the main Labour Force Survey (LFS) with additional data collected on a local sample boost. Apart from employment and unemployment, the topics covered in the survey include housing, ethnicity, religion, health and education.

The APS is a sample survey of households. There are approximately 320,000 persons per dataset.

Area of destination or origin within the UK

This is a migrants intended (immigration) or former (emigration) residence within the UK. These data are shown by countries of the UK (Scotland, Wales and Northern Ireland), and regions of England; North East, North West, Yorkshire and The Humber, East Midlands, West Midlands, East of England, London, South East and South West.

British citizenship

The countries for which the migrant is a British passport holder can include the United Kingdom and the crown dependencies of Jersey, Guernsey and the Isle of Man and 12 British territories: Anguilla, Bermuda, British Indian Ocean Territory, British Virgin Islands, Caiman Islands, Falkland Islands/British Antarctic, Gibraltar, Montserrat, Pitcairn Islands, South Georgia and South Sandwich Islands, St Helena/Ascension, Tristan da Cunha, Turks and Caicos Islands. British Overseas is also a citizenship.


An estimation procedure that constrains sample-based estimates of auxiliary variables to known totals (or accurate estimates). Calibration is used to improve the regional distribution of immigrants. Further information can be found in Long-Term International Migration estimates methodology.

Countries and country groups

A range of different country groupings are used in the Office for National Statistics migration statistics. The various definitions and groupings are defined in the notes accompanying the tables. Unless otherwise stated, country groups in tables are presented as they were constituted on 1 May 2004.

In 2014, we ran a user consultation on country groups used in the reporting of international migration statistics. A response to the consultation was published and as a result, new country groupings replaced old country groupings in the Provisional quarterly estimates and annual final tables. Citizenship Groupings, Country of Birth Groupings and Country of Residence Groups are presented in the International migration – table of contents.


This is the term used in the International Passenger Survey (IPS) to define the country for which a migrant is a passport holder. This refers specifically to the passport being used to enter or leave the UK at the time of interview. It does not refer to any other passport(s) which migrants of multiple nationalities may hold. More generally a British citizen as described in IPS statistics includes those with UK nationality usually through a connection with the UK: birth, adoption, descent, registration, or naturalisation. British nationals have the right of abode in the UK.

Confidence interval

This is the range within which the true value of a population parameter lies with known probability. For example, the 95% confidence interval represents the range in which, over many repeats of the sample under the same conditions, we would expect to find the true value 95 times out of 100. Equivalently, we can say that there would be a 1 in 20 chance that the true value would lie outside of the range of the 95% confidence interval. The uppermost and lowermost values of the confidence interval are termed “upper and lower confidence limits”. Further information can be found in Long-Term International Migration estimates methodology.

Country of birth

The country in which a person was born.

Country of usual residence

Based on the UN definition, the country in which a person has a place to live, where he or she normally spends the daily period of rest. Temporary travel abroad for purposes of recreation, holiday, visits to friends and relatives, business, medical treatment or religious pilgrimages does not change a person’s country of usual residence.


The European Economic Area (EEA) consists of the 28 countries of the EU (see next definition), plus Iceland, Liechtenstein and Norway. Swiss nationals are treated as EEA nationals for immigration purposes; however, for statistical purposes Switzerland is not included in EEA estimates by ONS unless otherwise specified. European Union

A list of which countries are in each country group is available in the international migration table of contents.


A person who leaves their country of usual residence to go to another country.

Emigration (outflow)

The number of people leaving the UK.

Illegal or irregular migrants

The London School of Economics identified three main categories of illegal or “irregular” migrants, which include:

  • illegal entrants – including both those who evade formal migration controls and those who present false papers

  • migrants who have been lawfully present in the country but remain after the end of the permitted period (two main subcategories)

  • failed asylum seekers who stay in the country despite a final decision refusing them continuing right to remain

  • overstayers whose period of legal residence has expired without renewal: this group includes those who are no longer eligible to apply for extensions because of the introduction of the points system

  • children born in the UK to irregular migrant couples; they are not migrants themselves, but have no right to remain

By its very nature it is impossible to quantify accurately the number of people who are in the country illegally. For this reason, Office for National Statistics (ONS) do not produce estimates on the size of the illegal migrant population. However, while our data do not identify illegal migrants separately, many will be included in our data. Those who overstay their visa would have been counted in our immigration figures by the International Passenger Survey (IPS) when they originally entered the country and those who arrived illegally and then subsequently claimed asylum will also be included.

Furthermore, every 10 years ONS conduct the census and at this time a notable proportion of the illegal migrant population should be captured in the population estimates. The 2011 Census initially captured 94% of the resident population using an address register and focusing on hard-to-count areas. Adjustments were then made following the Census Coverage Survey to estimate the whole population.

In June 2005, the Home Office published the outcome of an assessment of whether methods used in other countries to estimate the size of the illegal population could be applied to the UK. The outcome estimated that the total unauthorised migrant population living in the UK in 2001 was 430,000. Please see the following reports for more information: 29/05 – Sizing the unauthorised (illegal) migrant population in the United Kingdom and 58/04 – Sizing the illegally resident population in the UK.


A person arriving or returning from abroad to another country.

Immigration (inflow)

The number of people coming to the UK.

International Passenger Survey (IPS)

The International Passenger Survey (IPS) is a survey of a random sample of passengers entering and leaving the UK by air, sea or the Channel Tunnel. Between 700,000 and 800,000 people are interviewed on the IPS each year. Of those interviewed, between 3,000 and 4,000 people each year are identified as long-term international migrants.

Labour Force Survey (LFS)

Labour Force Survey (LFS) – a quarterly household survey run by Office for National Statistics (ONS).

Long-term international migrant

ONS uses the UN recommended definition of a long-term international migrant: “A person who moves to a country other than that of his or her usual residence for a period of at least a year (12 months), so that the country of destination effectively becomes his or her new country of usual residence.”

This is the definition used to calculate net migration, and is also used for the UK usually resident population estimate series. This definition does not necessarily coincide with those used by other organisations. Table 6 in the International Passenger Survey: quality information in relation to migration flows compares how other countries define and measure long-term international migration in comparison to the UK.

Long-Term International Migration (LTIM) estimates

Long-Term International Migration (LTIM) estimates are produced by combining migration data from the International Passenger Survey (IPS), Home Office data on asylum seekers (including non-asylum enforced removals adjustment and people resettled in the UK under various resettlement schemes), migration to and from Northern Ireland (from the Northern Ireland Statistics and Research Agency) and adjustments for visitor switchers and migrant switchers.


In-depth documentation, detailing the processes behind the international migration statistics produced by ONS.

Migrant switchers

Travellers who stated the intention in the International Passenger Survey (IPS) to stay in the destination country for at least a year, who were therefore counted as migrants, but who actually left sooner.

National Insurance number (NINo)

National Insurance numbers are issued to non-UK nationals immigrating for work by the Department for Work and Pensions (DwP). The number of NINos will include people who are coming to the UK for short periods or temporary purposes, as well as long-term migrants. The figures are based on recorded registration date on the National Insurance recording and Pay As You Earn system (NPS), that is, after the NINo application process has been completed and so are not a direct measure of when a person migrated to the UK.


Nationality is often used interchangeably with citizenship, and some datasets refer to “nationals” of a country rather than “citizens”. Different datasets have different ways of establishing someone’s nationality. The Annual Population Survey (APS), which underlies the population estimates by nationality, simply asks people “what is your nationality”. However, the International Passenger Survey (IPS), National Insurance number (NINo) and entry clearance visa data are based on people’s passports. For asylum statistics, the nationality is as stated on the “Case Information Database”. This will usually be based on documentary evidence, but sometimes asylum seekers arrive in the UK without any such documentation.

Net migration (balance)

The difference between the number of people coming to live in the UK (immigration) and the number of people leaving to live elsewhere (emigration). When more people are arriving in the UK than leaving, net migration is above zero and so adds to the non-UK population.

Non-sampling error

Non-sampling errors are attributable to all other sources than sampling. These may include misunderstanding or misreporting by respondents, variations between the way interviewers administer the survey, non-coverage of the population due to an inadequate sampling frame or sample design and errors made when processing. Further information can be found in Long-Term International Migration estimates methodology.

Non-visa nationals

Non-visa nationals are people who do not require a visa to enter the UK. European Economic Area (EEA) nationals do not normally require a visa to enter the UK (although a small number of EEA nationals do apply and are issued visas). Additionally, for those non-EEA nationals classified as “non-visa nationals” (for example, citizens of the USA, Brazil and Japan) a visa is not normally required for visits of less than six months.

There are two ways in which non-visa nationals can enter the UK for work and study purposes without a visa. Non-visa nationals are allowed to work in the UK without a visa, but only for sporting or creative work of less than three months duration, this falling outside the scope of the UN definition of a short-term migrant. For study, there are many more admitted into the UK under the short-term study visa scheme than visas issued, as non-visa nationals are allowed to study under the scheme for up to six months without a requirement for a visa.

Quality assurance

Processes carried out by ONS during the production of international migration statistics to ensure the quality of the statistics. These processes are detailed in the QMI documents.

Points Based System (PBS)

The Points Based System (PBS) is a rationalisation of immigration control processes for people coming into the UK for the purposes of work or study who are not European Economic Area (EEA) or Swiss nationals. Entries are classed into five tiers:

  • Tier 1 is for high-value individuals such as investors and entrepreneurs
  • Tier 2 is for skilled workers with a job offer
  • Tier 3 is low skilled workers – this entry route was never opened and is currently suspended
  • Tier 4 is for students
  • Tier 5 is for youth mobility and temporary workers

Provisional data

Provisional estimates of Long-Term International Migration (LTIM) give a timely indication of the flow levels as they are available five months after the reference period. Provisional data are later revised to become final estimates. The Long-Term International Migration estimates methodology document explains the difference between provisional and final IPS and LTIM estimates and compares the latest figures.

Rolling year

Any four consecutive quarters that makes up a 12-month period. In the provisional tables, the rolling year moves on one quarter for each row of estimates in the tables. For example, the rolling year January 2009 to December 2009, is followed by the rolling year April 2009 to March 2010.

Sampling error

The difference between an estimate derived from a random sample and the true population value; the difference being attributable to the fact that only a sample of values was used. That is, sampling error results because not every migrant who enters or leaves the UK is interviewed. Further information can be found in Long-Term International Migration estimates methodology.

Short-term international migrant

The UN definition of short-term migrants relates to those who change their country of usual residence for between three and 12 months for the purposes of work or study. In line with this UN definition, the analysis of visas presented in the Short-Term International Migration bulletin relates solely to those who have arrived in the UK for work or study on short-term visas.

We also publish short-term international migration statistics for those migrating for three to 12 months for all reasons for migration and for one to 12 months for all reasons for migration.

Short-term visa

A short-term visa is defined as being a visa of less than 12 months duration. It is important to recognise that visa duration does not necessarily represent the length of stay, as many individuals will depart prior to the expiry date. Individuals may also be granted extensions of stay. It is, therefore, not possible to determine whether a holder of a visa has actually stayed for three months or more.

Most short-term student visas are issued for a fixed six months duration, or in some limited conditions for 11 months (with no possibility of extension in either case); because visa duration does not necessarily represent the length of stay, it is not possible to make any inference as to whether an actual visit exceeded three months from the six months fixed visa duration. Similarly, short-term visas issued for work and for regular study are likely to reflect an individual’s initial planned length of stay, with the actual length of stay potentially being longer due to extensions for work placements or study courses.

The figures presented in the Short-Term International Migration report relate to visas issued for entry clearance to the UK and are not related to individual countries within the UK. The figures include dependents.

Standard error (SE)

An indication of the accuracy of an estimate and how much a sample estimate is likely to differ from the true value because of random effects. Further information can be found in Long-Term International Migration estimates methodology.

Statistical significance

The International Passenger Survey (IPS) interviews a sample of passengers passing through ports within the UK. As with all sample surveys, the estimates produced from them are based upon one of a number of different samples that could have been drawn at that point in time. This means that there is a degree of variability around the estimates produced. This variability sometimes may present misleading changes in figures as a result of the random selection of those included in the sample. If a change or a difference between estimates is described as “statistically significant”, it means that statistical tests have been carried out to reject the possibility that the change has occurred by chance. Therefore, statistically significant changes are very likely to reflect real changes in migration patterns. Further information can be found in Long-Term International Migration estimates methodology.


Estimates for study refer only to migrants arriving to or leaving the UK for formal study.

Visitor switchers

Visitors who stated the intention in the International Passenger Survey (IPS) to enter or leave the UK, intending to stay in the destination country for less than a year, who were therefore counted as short-term migrants or visitors, but actually stay for 12 months or more.

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Contact details for this Methodology

Nicola White
Telephone: +44 (0)1329 444097