1. Introduction

1.1 Purpose

The aim of this article is to provide a description of the methodology used to calculate final estimates of Long-Term International Migration (LTIM) for the period since 1991. The current methodology was introduced in 2009 for the calculation of the 2008 estimates and revisions were made to earlier years as appropriate. This article also sets out previous methodological changes made to the series since it was introduced.

1.2 Definition of a migrant

Office for National Statistics (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.”

page 18, Recommendations on Statistics of International Migration

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.

1.3 Issues with measuring migration

There is no single, all-inclusive system in place to measure all movements of people into and out of the UK. Therefore it is necessary to use a combination of data from different sources, which have different characteristics and attributes, in order to produce estimates of international migration. . These sources are described and discussed in the Migration Statistics Quarterly Report – information for users.

Measuring illegal migrants presents another challenge. The London School of Economics1 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; this category includes 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 we 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 we 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 focussing 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. As mentioned previously a report written by the London School of Economics1 estimated that in 2007 the number of "irregular" migrants was 618,000 (which includes all of the above listed main categories)2.

Notes for: Introduction

  1. 'Economic impact on the London and UK economy of an earned regularisation of irregular migrants to the UK', London School of Economics, (Greater London Authority, 2009)

  2. A more recent report has been published by Migration Watch who have updated the LSE report based on several different assumptions

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2. Current methodology

The methodology outlined in this section was first applied in 2009 for the calculation of the 2008 estimates and revisions were made to earlier years as appropriate.

As a consequence of these methodological improvements, the Long-Term International Migration (LTIM, formerly known as Total International Migration or TIM) back series was revised. The impact of these revisions can be seen in the Appendix.

2.1 Data sources used to compile final estimates of Long-Term International Migration (LTIM)

Estimates of LTIM are produced from these main data sources:

  • International Passenger Survey (IPS)

  • Labour Force Survey (LFS) – provides a geographical distribution of migrants for the calibration methodology (see Section 2.3)

  • Home Office data on asylum seeker flows and their dependants, on non-asylum enforced removals and on people resettled in the UK under various resettlement schemes

  • forecasted Long-Term International Migration estimates based on previous GP registrations from the Northern Ireland Statistics and Research Agency (NISRA) for estimating long-term international migration to and from Northern Ireland and the Rest of the World, from 2008 onwards; forecasted data is replaced with final data for LTIM final annual estimates

2.2 Components of LTIM

LTIM comprises of a number of components, which are described in the following method:

2.3 International Passenger Survey (IPS)

The International Passenger Survey (IPS) is a sample survey of passengers arriving at, and departing from, UK air and sea ports and the Channel Tunnel. It is carried out by the Office for National Statistics (ONS) for a range of public and private sector organisations. In particular, it provides figures used for the travel account of the balance of payments, captures data on international tourism as well as providing data on the numbers and characteristics of short-term and long-term international migrants. Figure 1 shows each port at which the IPS interviewing currently takes place.

The long-term international migration data from the IPS is the largest component of LTIM. It is important to note that these data are intentions-based, for example, the survey asks how long each migrant intends to remain in or out of the UK, as opposed to recording what they have done on their later journeys.

In 2009, adjustments were made to the methodology of the IPS, in terms of both sampling and data processing. This resulted in a sample design that is better optimised for collecting data on migrants.

Please note that net migration estimates for 2001 to 2011 have been revised in light of the results of the 2011 Census, which showed that net migration was higher than implied by published migration estimates. A review into the Quality of LTIM estimates showed that the IPS missed migration flows, particularly of EU8 citizens, prior to improvements to the survey in 2009 that increased its coverage of regional airports.

The Appendix provides further detail on the IPS, including its sample design; how the collected data are weighted to be representative of the total numbers travelling; information about the quality of the estimates; and details of the recent changes and their impacts. Further general information about the IPS can be obtained from the annual report Travel Trends – A Report on the International Passenger Survey.

A copy of the 2016 version of the IPS questionnaires can be found on the Data Archive website.

The IPS has some limitations with respect to measuring immigration and emigration, as it:

  • is a sample survey and so only a sample and not every migrant to or from the UK is interviewed; as a result, the estimates are subject to a degree of uncertainty

  • does not capture all asylum seekers who may be entering or leaving the UK

  • does not take into account the changing intentions of passengers (those who intended to remain in or out of the UK for 12 months, but actually spent less than a year and those who believed they would be staying or leaving for less than a year but actually spent longer)

  • does not capture those who are crossing the land border between the UK (Northern Ireland) and the Republic of Ireland

The IPS asks long-term immigrants to state where they intend to move to within the UK. Our research, as part of the Migration Statistics Improvement Programme (MSIP), compared IPS data with the 2001 Census and the Labour Force Survey (LFS) (a sample survey of households living at private addresses in Great Britain). This revealed that there are some migrants who will live at their intended destination for only a short period of time before moving elsewhere. In particular, IPS data show a greater proportion of migrants stating London as their destination compared with either the LFS or census data. One explanation is that London is an international gateway to the UK and is therefore a transition point before they settle in other parts of the UK.

The geographical distribution of immigrants who were recorded entering the UK by the IPS can therefore be improved with the use of the LFS. This is because it asks respondents where they lived a year ago and this identifies recent migrants. The LFS can therefore provide more reliable data on where migrants actually live rather than on their intentions when they first arrive.

A methodology has been developed that adjusts the IPS data to the geographical distributions provided by the LFS (known as “calibration”) and is described in detail in the article The use of calibration in estimating international in-migration to UK countries and the regions of England. The main steps are as follows:

  • LFS data are used to identify the geographical distribution of recent immigrants (those that arrived in the UK within the last year) by UK constituent countries and regions

  • these distributions are applied to IPS inflows to create a “control total” for each geographical area

  • IPS data are calibrated to each control total

An IPS dataset is created, which has the same total inflows as the original, but the estimates by geographical area are consistent with LFS data on where recent migrants are living.

Calibration is applied to individual IPS contacts, potentially changing the weight of each contact so that regional proportions match those of the LFS. Calibration can therefore affect all IPS breakdowns (for example, citizenship) not only regional breakdowns.

Outflow data are not put through the process of calibration and remain unchanged. An assessment of the impact of these changes on the LTIM series can be seen in Impact of revised methodologies on total international migration (TIM) estimates.

This improved methodology has been implemented back to 1999 because the scope of the original research only went back this far. Prior to this, the IPS alone was used to distribute migrants around the UK. Care therefore needs to be taken when examining detailed breakdowns of the IPS estimates before and after 1999, particularly when comparing regional or country estimates before and after this point.

2.4 Migrant and visitor switchers

As mentioned in Section 2.3, one of the main features of the IPS estimates is that they are based on passengers’ intentions. The IPS classifies long-term international migrants as travellers who intend to change their country of residence for at least a year. This can be either overseas residents arriving to live in the UK, or UK residents leaving to live abroad.

Sometimes these intentions may not be realised. People who enter or leave the UK intending to be a visitor, that is staying or being away for less than 12 months, may actually migrate for more than a year. These people are, in effect, visitors who subsequently become migrants and are referred to as “visitor switchers”. These migrants must therefore be added to the estimate of migration to make it comprehensive.

Alternatively, some people who enter or leave the UK intending to migrate (for 12 months or more), may actually stay in or leave for less than a year. These people are known as “migrant switchers” as they intended to be migrants, but were actually visitors. They need to be removed from IPS migrant flows. These adjustments improve the accuracy of the LTIM estimates.

These switchers are identified by the IPS as they complete their journey when subsequently entering or leaving the UK. The passenger is asked how long they intended to stay in the UK or overseas when they initially arrived or departed and for how long they actually remained in or out of the UK.

Visitor switcher methodology

Travellers who intend to stay in or leave the UK for less than 12 months are recorded by the IPS as “visitors”. It is known that a proportion of visitors, who state an intention to stay in their destination country for 6 to 12 months or possibly 12 months, could potentially stay for more than 12 months and therefore subsequently become migrants. These people are known as “visitor switchers”.

In response to a need for more accurate estimates of visitor switchers, new IPS questions were introduced in 2004. These questions collect data on respondents who did not intend to stay in or leave the UK for longer than a year, but subsequently did. These data are then used to provide a more informed indication of how many visitors will change their intentions and become migrants. This is an improvement to the previous methodology, which estimated how many of the potential visitor switchers would become migrants, without the additional information from the IPS (see Section 3.0).

It is known that the likelihood of a visitor changing their intentions can vary depending on their citizenship and place of last or next residence. To take these differences into account, the visitor switchers are split into four groups before any calculations are carried out: those entering the UK who are EEA and non-EEA citizens, those leaving the UK who are EEA citizens going to the EU, and all “other” citizens leaving the UK going to anywhere in the world. (The EEA refers to the European Economic Area, which is the EU plus Iceland, Liechtenstein and Norway.)

For each group the following calculation is made:

For details of the proportions of components that make up LTIM estimates, please see Table 1.01 for long-term migrants.

Migrant switcher methodology

The new IPS questions introduced in 2004 also collect data that can help improve the estimation of the number of migrant switchers. As with visitor switchers, these questions gather information on a traveller’s completed journey, therefore allowing the estimate to be calculated using actual migrant switcher data, as opposed to just using data for those who originally intended to be migrants.

As with the calculation of visitor switchers, a fraction is produced that takes the number of migrant switchers (over the previous 3 years) and divides these by the number of migrants recorded by the IPS in the previous 3 years. This denominator is therefore the pool of travellers who could potentially become migrant switchers as they were initially recorded stating an intention to be migrants. It is produced separately for both immigration and emigration. Unlike visitor switchers, there is no distinction between citizenships or countries of last or next residence for migrant switcher calculations.

The number of migrant switchers is then removed from the estimate of LTIM in the reference year as these people are not migrants. The proportion will change each year depending on the number of both migrants and migrant switchers captured by the IPS.

For details of the proportions of components that make up LTIM estimates, please see Table 1.01 for long-term migrants.

Due to the new IPS questions being introduced in 2004, a decision was made to apply the new methodology to the 2004 estimates onwards. Care therefore needs to be taken when comparing LTIM estimates before and after this year. The 2006 calendar year estimates were the first to use a full 3 years of data as required by the methodology as the new questions in the IPS were only introduced in 2004.

A comparison of how the fractions have changed using the previous and current methodologies are provided in Appendix C of Impact of revised methodologies on total international migration (TIM) estimates.

2.5 Asylum seekers and non-asylum enforced removals

As mentioned in Section 2.3, the IPS does not interview all asylum seekers entering or leaving the UK. In order to produce LTIM, we obtain data from the Home Office (as they are responsible for immigration control and applications for settlement, citizenship and asylum) on principal applicant asylum seekers and their dependants. Details can be found on the Home Office (UK Visas and Immigration Agency) website.

Data are provided for different types of asylum seekers. This includes the number of those who applied for asylum, were refused asylum, appealed against their asylum decision, asylum seekers who were returned home and those who withdrew. These different categories dictate whether the asylum seeker is leaving or entering the UK. An adjustment for asylum seekers returned, departing voluntarily, or withdrawing their application and leaving the UK within a year of the application, is made. This therefore excludes those who are not long-term migrants from the LTIM estimates.

Asylum applications covered by the Home Office can be identified as either “port” or “in-country”. Port asylum seekers – the minority – are those who apply at port when entering the UK. Most port asylum seekers are not captured in the IPS because they are usually escorted over the IPS counting line. An allowance is made when estimating port asylum seekers for the small number of migrants in the IPS data who give “seeking asylum” as their reason for entry and will therefore be double-counted if kept in.

In-country asylum seekers are those who enter the UK and later apply for asylum while in the UK. It is assumed that information about planned duration of stay given to the IPS interviewer is the same as that given to the Immigration Officer and, therefore, that in-country asylum seekers are unlikely to be captured as migrants in the IPS.

In summary, asylum seeker immigration figures are based on the number of people applying for asylum. This data is used to adjust the IPS estimates in order to:

  • exclude those asylum seekers counted by the IPS on arrival in the UK to remove the possibility of double-counting

  • allow for the small numbers of those counted in both the principal applicant and dependant applications data

  • exclude those who were returned within a year of their application

Asylum seeker emigration figures are based on:

  • the number who were returned to their country of origin

  • the number who withdrew their application and were known to have left the UK

  • a small number of applicants who had been refused asylum in the previous year (and, if appropriate, had been unsuccessful at appeal) or who had withdrawn their application and were not known to have left the UK.

The Home Office also collects data on non-asylum enforced removals – these are people who have been removed from the UK and who have not claimed asylum at any stage. They would not be interviewed by the IPS upon leaving the UK as they would not cross the sampling line. No adjustment for these people is required for inflow estimates, as they were not asylum seekers and therefore would have crossed the IPS sampling line on entering the UK.

For 2013 data onwards, it is possible to identify long-term migrants within the data on non-asylum enforced removals. Therefore for 2013 estimates onwards, an adjustment is made to include non-asylum enforced removals in asylum seekers' emigration estimates. The approximate impact of applying the adjustment is to increase emigration estimates by 2,000 to 3,000 per year and reduce net migration by around 1%.

For 2015 data onwards, adjustments using Home Office data on people resettled in the UK under various resettlement schemes have been included within LTIM estimates. This affects the inflow and, as a result, net balance estimates for Long-Term International Migration.

2.6 Estimate of migration to and from Northern Ireland

As mentioned in Section 2.3, the IPS does not sample those passengers who cross the land border between the UK (Northern Ireland) and the Republic of Ireland. In addition, no ports in Northern Ireland have historically been surveyed in the IPS, although this started at Belfast International Airport in 2009.

Family doctor registration data is the most complete source that can be used to estimate international immigration to Northern Ireland. This source gives information on an intention to stay for a period of time and covers all age groups.

The health card system records de-registrations with family doctors in Northern Ireland, while the Central Statistics Office (CSO) Ireland Quarterly National Household Survey provides the number of people moving from Northern Ireland to the Republic of Ireland. In combination, these sources are used to estimate emigration from Northern Ireland to all countries outside the UK.

These estimates are then incorporated into the LTIM estimates. A more detailed explanation of this methodology and the recent changes is available in Improving estimates of international migration in Northern Ireland, and between the UK and the Republic of Ireland. Further information about international migration statistics for Northern Ireland is available at the Northern Ireland Statistics and Research Agency (NISRA).

It should be noted that since 2014, we use forecasted data provided by NISRA for quarterly LTIM estimates. This is to improve timeliness of statistical outputs. Forecasted data is replaced with final data for the annual final LTIM estimates.

2.7 Assumptions made in order to produce LTIM

The published LTIM figures are broken down to show estimates by variables such as citizenship and age and sex. To produce estimates for each of these variables, data from the sources that contribute to LTIM also need to be broken down by the same variables.

Migrant data from the IPS is available broken down by each variable. Data on Northern Ireland flows and asylum seeker data are not, and need to be derived using a series of assumptions. In addition, the IPS data used to calculate the visitor switcher adjustments are based on a relatively small sample size each year, but still need to be broken down in the same way.

The following tables detail how the Northern Ireland flow data, asylum seekers and visitor switcher data are broken down for each variable. It is not necessary to do further processing to form assumptions for the migrant switcher data as it is applied as a direct proportion of the IPS migrant estimates.

2.7.1 Citizenship (and country of birth) assumptions

2.7.2 Country of last or next residence assumptions

2.7.3 Main reason for migration assumptions

2.7.4 Usual occupation (prior to migration)

2.7.5 Origin or destination distribution within the UK, assumptions

As discussed in Section 2.3, IPS data are adjusted using the LFS distributions to more reliably distribute immigrants throughout the UK, known as calibration. The following assumptions are made for people who are not covered by the IPS or whose intended length of stay changes:

2.7.6 Age and sex assumptions

2.7.7 Sex and marital status assumptions

2.7.8 Intended length of stay assumptions

A distribution for intended length of stay from the IPS is used for those entering and leaving the UK for work or study.

2.8 Provisional LTIM estimates

Provisional estimates give a timely indication of the flow levels of international migration, being produced 5 months after the reference date, compared with 11 months for final data. For LTIM they are produced quarterly on a rolling year basis. Top level figures, in addition to estimates by citizenship and reason for migration, are available. They are produced in exactly the same way as final LTIM estimates, except that some of the data sources are provisional. Previous time periods on the chart only use final data. More information on the differences between provisional and final LTIM data is available in: Frequently asked questions and background notes for long-term international migration estimates.

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3. Summary and timeline of methodological changes

Over time small adjustments have been made in the methodology to produce LTIM estimates. These reflect improvements in the components or statistical techniques used to estimate the flow of international migrants. Table 9 summarises changes, whereas Table 10 identifies where discontinuities exist in the present time series of LTIM, from 1991 to the latest time period.

Table 10: Discontinuities in the component data of final LTIM estimates

Period International Passenger Survey (IPS) Data on flows to and from Northern Ireland Data on flows to and from the Republic of Ireland Visitor Switchers Migrant Switchers Asylum Seekers
1991 Data from the IPS Data from Central Statistics Office of Ireland. Estimated using fixed proportions of EEA and non-EEA citizens to EU and Other countries. Estimated that 5% of inflow of IPS migrants and 1% of outflow become Migrant Switchers each year. Calculated by ONS using Home Office Data.
1992
1993
1994
1995
1996
1997
1998
1999 IPS inflow calibrated to LFS distribution of recent migrants.
2000
2001
2002
2003
2004 Visitor Switcher calculations based on information from new IPS questions on actual length of stay. Migrant Switcher calculations based on information from new IPS questions on actual length of stay.
2005
2006
2007
2008 Data supplied by NISRA using family doctor registrations. IPS data used.
2009 IPS inflow calibrated to LFS distribution of recent migrants.

Changes to sample design and data processing.
2010
2011
2012
2013 Adjustment for non-asylum enforced removals included.
2015 Adjustment for people resettling in the UK under various resettlement schemes.
Source: Office for National Statistics


Unfortunately it is not always possible to apply changes in methodology to the entire back series. This is because of problems regarding data availability in earlier years. This has resulted in unavoidable discontinuities in the time series. Overall these discontinuities are small and it is important not to confuse the size of the revisions with the real underlying trends in LTIM.

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4. Appendix: Further information on the International Passenger Survey (IPS)

IPS Sample design

Background

The IPS uses a multi-stage sample design, which is carried out separately for air, sea and tunnel travel. The underlying principle is that, in the absence of a sampling frame of travellers, time shifts or sea crossings are selected and then travellers are systematically chosen at fixed intervals within these shifts or crossings.

At the airports a certain number of shifts are sampled randomly each quarter, each stratified by time of day and by day of the week. Passengers are counted as they cross a predetermined line and every nth person is interviewed. At sea ports, passengers may be sampled on the quayside as they embark or disembark. The sampling approach is similar to that at the main airports as the timing of the interviewing shift is selected randomly. At other ports, interviewers travel on the boats and sample passengers systematically. For tunnel routes, the method is different for passenger trains and vehicle shuttles. Passenger trains are treated in a similar way to airports, where time shifts are selected and then a sampling interval used within a time shift. In contrast, for vehicle shuttles, crossings are randomly selected and interviewing takes place on board the shuttles themselves.

Prior to 2009

Prior to 2009, the main UK airports – Heathrow, Gatwick and Manchester – were always included in the sample. Other airports were included in the sample if they had at least 1 million passengers a year passing through them. Sea routes were treated similarly to the smaller airports, in that they were included or excluded in the sample depending on the size of the international traffic passing through.

Extra samples, referred to as migration filter shifts, were also carried out on the inward (arrival) flows at the five Heathrow and two Gatwick terminals to boost the sample of migrants. These were first run in departures (outflow) in 2007. Contacts were asked a brief series of questions to identify whether or not they were migrants and only migrants were given a full interview.

Changes made in 2009

In January 2009, changes were made to the sample design and data processing of the IPS. These changes were introduced following a Port Survey Review in response to the recommendation put forward by the Inter-Departmental Task Force on Migration Statistics in 2006. Further information on the Port Survey Review is available in Port Survey Review - Stage Two Final Technical Report

The sample design changes consisted of:

  • increasing the number of shifts run at many air or sea or tunnel ports apart from at Heathrow

  • decreasing the number of shifts run at Heathrow

  • new shifts being established at Aberdeen and Belfast airports and the Portsmouth to Bilbao sea route

  • abolishing the migration filter shifts

  • introducing a primary sampling interval for screening migrants (around 1:10) and a sub-sample interval for travel and tourism contacts (around 1:30) within the ordinary shifts, therefore travel and tourism contacts are a sub-sample of the migrant sample

The new sample design aims to be both more migrant focused and more balanced in terms of the routes that migrants use. An optimisation exercise was undertaken using 2006 IPS data to determine the distribution of IPS shifts by route that would be expected to deliver the most statistically accurate sample.

Grossing the IPS interviews to total numbers travelling

Sample contacts need to be grossed to represent total estimates. This is done by using a complex weighting system. The method of grossing the interviews to national estimates varies depending on the method of travel.

The 2009 changes included improvements to the weighting and imputation of IPS records. A detailed description of how the IPS raw data is grossed is available in Travel Trends – A Report on the International Passenger Survey

Quality of the IPS survey data

The IPS is a sample survey and is, therefore, subject to some uncertainty. Figures obtained from the IPS are subject to both sampling and non-sampling errors.

Sampling error

Sampling error arises due to the variability that occurs by chance because a sample, rather than an entire population, is surveyed; that is, sampling error results because not every migrant who enters or leaves the UK is interviewed. Sampling errors are determined both by the sample design and the sample size. Sampling error may sometimes present misleading changes as a result of the random selection of those included in the sample.

Confidence intervals (CI) are provided with IPS-based estimates and are a statistical method by which sampling error can be measured. A confidence interval 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 the confidence interval to contain 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 “confidence limits”. You are advised to be cautious when making inferences from estimates with relatively large confidence intervals. For immigration and emigration estimates where the lower confidence interval is below zero you should assume the estimate is above zero.

When estimates are broken down to lower levels of detail, greater care must be taken with their interpretation. This is because these estimates will be based on a smaller number of survey contacts, which increase the uncertainty around the estimate. For example, it is not possible to produce estimates for most individual citizenships or countries of last or next residence, within a single year, because of the small number of survey contacts that comprise each estimate.

Even where the sample size allows individual country estimates to be produced, it is often not possible to say that a change in the estimate from one year to the next is real or not. This is because smaller estimates often have proportionately larger confidence intervals than larger estimates.

Details of the possible effects of sampling error on the migration estimates by various characteristics are given in Table 1.02 of the Long-Term International Migration tables. Entries in this table show that estimates based on the sampling of passengers on certain routes have proportionately larger confidence intervals associated with them. Thus, generally speaking, the reliability of the estimate increases in proportion to the size of the estimate.

Statistical significance

Confidence intervals are also useful when comparing differences between estimates. 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 hypothesis that the change has occurred by chance. Therefore significant changes are very likely to reflect real changes in migration patterns.

A quick method of identifying if the difference between two estimates is statistically significant is to determine if there is an overlap of their confidence intervals. If they do not overlap, then the differences can be described as statistically significant. However, if they do overlap, then a t-test should be performed to determine significance.

This test divides the difference of the estimates by the square root of the sum of the squared standard errors. The standard error can be calculated by dividing the published confidence interval by 1.96. The resulting t-value needs to be greater than 1.96 to be 95% certain that the estimates are different. It can also be used to create a confidence interval around the difference. It calculates the standard error of the difference directly from using the difference between the two individual standard errors.

All main statistical software packages have the functionality required to perform a t-test. If you need assistance with identifying whether the difference between two international migration estimates is statistically significant then please contact migstatsunit@ons.gov.uk.

With regards to the use of t-tests with LTIM estimates, the additional data sources used in LTIM are not subject to the uncertainty associated with a sample survey, although it is recognised that they are unlikely to be error-free. There is no method of quantifying the possible error associated with the non-survey components of LTIM and these errors are unlikely to be random. Therefore statistical tests for significance are best applied to IPS-based estimates.

Non-sampling errors

Non-sampling error is all error that is not sampling error. The challenge with non-sampling error is that it is difficult to directly calculate a numerical measure of its effect. This, therefore, makes it hard to incorporate when analysing results. Non-sampling error is best understood by referring to examples that apply to the IPS.

The first non-sampling error may be due to non-response. Bias will occur when passengers who do not respond to the survey have different characteristics to those who do respond. Possible low levels of response that might be expected due to the respondent not speaking English have been reduced in recent years by the introduction of separate sampling arrangements at the Port Health Channel. This improvement is at least partly because interviewers can more easily enlist the help of relatives or interpreters to translate for contacts who do not speak English.

A further source of bias may arise from contacts deliberately concealing their migration intentions from the interviewers. In addition, the question that determines whether the contact is a migrant or not and their length of stay, is based on intentions and not actual behaviour. Measurement errors could therefore be introduced if there is a discrepancy between those intending to migrate, but who subsequently stay less than a year and those not intending to migrate, but who stay for a year or more.

For those contacts identified by the IPS as migrants, the level of non-response is very low for most characteristics. Latest details of survey non-response can be found in Appendix E of the latest annual report Travel Trends – A Report on the International Passenger Survey.

International Passenger Survey: Quality Information in Relation to Migration Flows provides an overview of the quality and reliability of the IPS in relation to producing estimates of long-term migration flows.

Impact of 2009 changes

Analysis carried out by Office for National Statistics (ONS) has shown that the 2009 changes to the collection of the IPS data have created some small discontinuities between the international migration estimates for 2008 and 2009. However, these are all within one standard error of the published figures and are no cause for concern. As a result of the changes the estimates being produced are now more statistically accurate, particularly for international migrants who use arrival and departure points around the UK.

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