The International Passenger Survey (IPS) is a large multi-purpose survey that collects information from passengers as they enter or leave the UK. It is carried out by the Office for National Statistics (ONS) for a range of public and private sector organisations. In particular, the survey provides figures used for the travel account of the balance of payments, international migration statistics, and for informing decisions on tourism policy.
The data from the survey are widely used across and outside of government to provide detailed information on the numbers and types of people travelling to and from the UK. Results are published regularly by ONS on a monthly, quarterly and annual basis. More detailed analyses are possible through the Data, Advice and Relations Team (DART) in ONS, or by downloading the travelpac (4.41 Mb ZIP) dataset from the ONS website.
Travellers passing through passport control are randomly selected for interview and all interviews are conducted on a voluntary and anonymous basis. Interviewing is carried out throughout the year. The overall response rate (complete and partial interviews) for the 2013 survey was 80%.
Since the IPS began in 1961, its coverage has been extended so that it includes all the main air, sea and tunnel ports or routes into and out of the UK. The only routes excluded from the survey are sea routes to and from the Channel Islands, the land border with the Irish Republic, and cruise ships travelling to and from the UK.
Approximately 95% of passengers entering and leaving the UK are covered by the survey. The remainder are either passengers travelling at night, when interviewing is suspended, or on those routes too small in volume or too expensive to be covered.
The IPS data are weighted to produce national estimates of all international travellers to and from the UK on a quarterly basis. Although some provisional monthly data from the IPS are also published, a single quarter is the minimum period over which most detailed analyses of the data can be made. Annual national estimates are created by combining the four quarters of the year.
The calculation of the weights on the IPS takes into account its complex sample design and information provided from other sources on, among other things, the non-sampled routes and time periods. For example, the Central Statistics Office in the Irish Republic provides information on travellers crossing the land border with Northern Ireland.
Overview of the survey design for collecting Overseas Travel and Tourism information
The IPS is based on face-to-face interviews with a sample of passengers travelling via the principal airports, sea routes and the Channel Tunnel. The number of interviews conducted to produce overseas travel and tourism estimates in 2013 was 296,000, this large sample size allows reliable estimates to be produced for various groups of passengers despite the low proportion of travellers interviewed.
The IPS sample is stratified to ensure it is representative by mode of travel (air, sea or tunnel), port or route, and time of day. The frequency of sampling within each stratum is varied according to the variability of tourist expenditure and the cost of interviewing. For example, where the expenditure quoted on a particular route varies greatly across respondents, a higher sampling frequency is used to enable a more satisfactory estimate to be produced. (For further details on the sample design, see the Sampling section and the IPS Overseas Travel and Tourism User Guide (Volume 1): Background and Methodology (423 Kb Pdf)).
Some questions on the survey are asked of all of the passengers interviewed, while others are restricted to certain specific sub-groups. Information on the spending and length of stay of UK residents abroad and overseas residents in the UK is only collected on the return leg of a visit. This is because actual spending and length of stay are required, and these may differ from the respondents’ intentions when they start their visit. In 2013 the sample on which the estimates presented in this publication are based included, 49,000 interviews carried out with overseas residents departing from the UK and 59,000 with UK residents arriving back from abroad.
The details collected on the survey are used by ONS, along with other sources of information, to produce overall national estimates of the number and expenditure of different types of travellers. A complex weighting procedure is used to do this that takes into account various factors in order to improve the estimates. (For further details of the weighting procedure, see the Producing national estimates section).
The collection of the IPS data
The key to producing reliable results from the IPS lies initially in the way the data are collected. Great emphasis is therefore placed upon the IPS interviewers to ensure they are able to capture data efficiently and accurately.
Nationally, IPS data are collected by a team of over 200 interviewers who are recruited and trained specifically to work on the IPS. Interviews are carried out on all days of the year, apart from Christmas Eve, Christmas Day and Boxing Day.
Almost all IPS interviews take place on a face-to-face basis with the responses being initially recorded on paper forms. In recent years ‘self completion’ questionnaires have been used at times where an interviewer has been unable to conduct an interview because of language difficulties.
Due to the layout and facilities at some seaports it is not always possible to interview passengers as they arrive. In such cases, IPS staff travel to seaports in France and Ireland to select their subject and then conduct interviews which take place either at the overseas ports of departure, or on board the vessels returning to the UK.
Shortly after the interview has taken place, the data are transferred to a computer system in which electronic checks are made of the data being input and the data is then transmitted to ONS headquarters where a series of further quality and accuracy checks are made on the data before processing and analysis.
More information about the collection of IPS data can be found in the IPS Overseas Travel and Tourism User Guide (Volume 1): Background and Methodology (423 Kb Pdf).
The IPS uses a multi-stage sample design. The sampling for air, sea and tunnel travel is carried out separately, although the underlying principle for each mode of travel is broadly similar. In the absence of a sampling frame of travellers, time periods/shifts or sea crossings are selected at the first stage (primary sampling unit), and travellers are then systematically chosen at fixed intervals from a random start within these shifts or crossings at the second stage. The details of the sampling scheme for each individual mode of travel are described below. More information about the IPS sample design can be found in the IPS Overseas Travel and Tourism User Guide (Volume 1): Background and Methodology (423 Kb Pdf) .
For air routes, time periods are sampled. Shifts are selected for the first stage. These are done in such a way that the numbers of shifts are balanced between mornings and afternoons, and days of the week within any quarter. At the second stage, passengers are counted as they cross a predetermined line and every nth person is interviewed.
The sampling interval, n, differs between sites and involves a first stage sampling rate used to screen respondents for migration purposes and a second stage sampling rate used for overseas travel and tourism interviews. Departing passengers are sampled at a higher rate than those arriving because the expenditure information for overseas residents visiting the UK is more variable than that for UK residents returning from visits abroad.
A small number of shifts every quarter are also conducted at other smaller international airports in the UK. However, the sample size is insufficient to provide accurate estimates for most of these airports individually. Those airports with less than about 250,000 passenger movements per quarter are usually excluded from the survey altogether on the grounds of cost effectiveness, but traffic at these sites is taken into account when producing national estimates.
Sea routes carrying 50,000 passengers a year or more are generally included in the IPS sample. At some seaports, passengers are sampled and interviewed on the quayside as they embark or disembark, while at others IPS interviewers travel on the boat itself with interviewing being carried out on board. The choice between interviewing on the quayside or on crossings is made on practical grounds such as cost, safety and permission.
Where interviewing is conducted on the quayside, the sample is designed to select shifts that are balanced across different days of the week and times of day within a quarter, with each individual shift covering several sailings. Where interviews are conducted on crossings, a predetermined number of return crossings are selected for each route, spread across time of day and day of week each quarter. As for air sampling, sea passengers are selected at fixed sampling intervals from a random start within each shift or crossing.
The IPS also sample also includes long haul ships capable of carrying more than 200 passengers arriving and leaving from Southampton.
The method used for the tunnel routes is different for Eurostar passenger trains and for Eurotunnel vehicle shuttles.
The method for passenger trains is similar to that for air travel; time shifts are selected and then passengers are selected at fixed intervals within the time shift. Passengers are interviewed after crossing a predetermined line at Ebbsfleet, St. Pancras, and Ashford International stations on arrival or departure.
In contrast, for vehicle shuttles, crossings are randomly selected and interviewing takes place on board the shuttles themselves. Because of time constraints, only a certain number of interviews can be carried out on any individual shuttle and the sampling interval used is therefore dependent on traffic volumes.
Producing national estimates
Once the information has been collected from respondents, the survey data are weighted to produce national estimates, which are then published on a monthly and quarterly basis as provisional estimates and final estimates published annually.
The basis of the weighting of IPS survey data is that the total set of respondents interviewed at a port or route is weighted up/calibrated to passenger traffic known to have passed through that port or route in the period in question. The known passenger traffic information is provided to the IPS team by CAA, Department for Transport, Eurostar, Eurotunnel, BAA and a number of airports themselves.
The weighting approach incorporates a number of stages which take account of all passengers selected for interview. Weighting is conducted for each port/route and direction of travel combination, employing the same principles at each one. The stages, listed in order of application, are as follows.
A Design weight is employed, to account for the probability of sampling this passenger using the first-stage sampling rate.
The calculation compares the number of shifts or crossings sampled (at each port/route and direction of travel combination) with the number of shifts or crossings that could have been sampled for that combination in the period. In addition it takes into account the first-stage sampling rate. For example, in a case where a contact was sampled at a port with the following details:
10 shifts were run in the period,
100 shifts could have been run in the period,
The contact was the sample employing a first stage sampling rate of 20 (that is, every 20th passenger was selected).
The Design weight for this contact would be 200, calculated as (100/10) x 20. As well as port/route and direction, this weight incorporates weekday or weekend, and am, pm or night as weighting strata.
A Non-response weight factor is employed to take account of contacts selected for interview but who were subsequently not interviewed, either because it was not possible to contact them or they refused to participate.
The weight is applied at each port/route and direction of travel combination and also incorporates weekday versus weekend as weighting strata. It involves uplifting ‘complete’ and ‘minimums’ cases by a factor calculated as:
The sum of weights applied to all ‘completes’, ‘minimums’ and ‘non-response’ records,
Divided by the sum of ‘completes’ and ‘minimums’ at that port/route and direction of travel combination.
A second Design weight is applied to account for the second-phase of the sample design and relates to the sub-sampling of non-migrants. The weight for this factor is simply equal to:
A weight factor is applied for discarding minimum respondents. Minimum interviews are discarded in this step of the weighting, with other cases weighted up to compensate. The purpose of applying this weight is that it is possible that the profile of minimums might be skewed to certain nationalities or residents of certain countries (for example driven by language difficulties meaning that only minimal information is provided to the interviewer).
This weighting step works to the same principle as the non-response weight. It utilises port/route and direction of travel as weighting strata.
Weighting to the sampling frame. Here the population (that is, passenger traffic) or the ports and routes covered by the sampling frame are used to weight the data. The population excludes interlining passengers (those neither entering nor leaving the UK from this port, that is, simply changing international flights) and out-of-hours traffic (that is, arriving or departing outside the hours covered by the IPS interviewing at that port). The weight is applied at each port/route and direction of travel combination.
Weighting for frame under coverage. This extends the above population weighting to compensate for not covering certain ports and times of day (out-of-hours traffic) in the survey sample. The weight utilises port/route and direction of travel as weighting strata and also incorporates region of the world that traffic has come from/gone to. The weight reflects the fact that flights to and from some parts of the world are more likely than others to arrive, or take off at night, when no interviewing is conducted at airports.
Weighting for observed imbalance. This step is used to correct an observed imbalance between the number of non-migrants entering and leaving the UK. These are applied as a series of fixed factors, relating to direction of travel, port/route and country/residence.
It has been noted that during 2009 and 2010 there was an increase in the proportion of respondents in the IPS overseas travel and tourism sample who are starting their visit compared to the proportion ending their visit. This proportion of the two types of traveller in the sample defines the estimates of travel and tourism.
There is no clear reason for this trend, ONS has taken steps to calibrate its overseas travel and tourism estimates with external data, notably estimates from surveying conducted at departure gates at main airports in the UK by the Civil Aviation Authority (CAA) and e-borders data. This work showed general consistency between the datasets with the result that the factors used in the imbalance weight have been retained. More information about the work undertaken to explore the imbalance (32 Kb Pdf) and planned future work can be found on the ONS website.
A final weight is applied, which combines each of the weighting stages listed above.
Where the responses for key items of interest are missing from the survey data for an individual record the values are imputed. Imputation is applied to the following items:
For each of length of stay, cost of fare and spend, a value is calculated for the survey record which had the information missing. The IPS employs a mean-value within class imputation procedure where the missing value is replaced with the average value for records with similar characteristics. The matching variables used for each of these items are:
Length of stay: Country of visit/visiting from; Purpose of visit.
Cost of fare: Port in UK travelling to/from; Overseas port travelled to/from; Month of travel; Operator.
Spend: Country of visit/visiting from; Duration; Purpose of visit.
Where the respondent has travelled on a package holiday, the cost of the fare is imputed and then deducted from the total cost of the package, and the residual cost (after removal of a percentage to cover travel agent fees) is assigned to expenditure.
Overseas residents staying in the UK are asked about their total expenditure in the UK. This information is then imputed across the towns stayed in, proportionate to the length of stay in each one. It is recognised that people tend to spend more when they stay in London than in other towns in the UK and therefore an uplift index is calculated and applied to the spend allocated to London in cases where the respondent stayed in both London and other towns in the UK.
In cases where an overseas resident hasn’t given details of all the towns in the UK they stayed in, an uplift is applied to towns stayed in by similar records, using the same principles as outlined above for the imputation of stay, fares and spend.
The number of travellers and their spending both have a clear seasonal pattern, with more visits and spending in the summer than in the winter. Statistical techniques are used by ONS with the package X-12-ARIMA to produce seasonally adjusted figures. These figures show visits and spending with an estimate for the seasonal component removed. They allow more meaningful comparisons to be made between months and quarters of the year and help to identify underlying trends.
More details on seasonal adjustment procedures can be obtained from the IPS Branch of ONS.
Usually, spending by overseas residents in the UK and UK residents abroad grows each year as the price of goods and services rise. Constant price figures are calculated by ONS to show real spending across years with the effects of price inflation removed.
For overseas residents’ expenditure in the UK, an index is created by splitting spending into its component parts (accommodation, meals and so on) using past IPS data and uprating these components by their related retail price indices. The resulting index is then used to rebase the overseas figures back to 1995 prices.
For UK residents abroad, spending is split by country of visit. Consumer price indices for particular countries are used with currency conversion rates to produce an index of price rises. The index is then used to rebase UK residents’ spending to 1995 prices.
Additional sources of data
The method above explains how the national estimates are produced based on the routes sampled on the IPS. Unfortunately, as the IPS does not cover all passenger routes, additional figures have to be obtained from other sources or estimates and added to the totals derived from the IPS. These additions are:
UK residents on cruises departing from or arriving at UK shores,
Channel Islands expenditure and receipts from tourism,
Rail fares purchased by overseas visitors to the UK and UK visitors abroad before the start of their visit, and
Estimates of travel across the land border between Northern Ireland and the Irish Republic, from the Irish Central Statistical Office.
Changes in methodology introduced in 2005
Due to a rapid growth in traffic, in 2005 two new residual airports, Liverpool and Prestwick were introduced into the IPS sample for the first time. The introduction of these two airports has some implications for the results of the IPS. The inclusion of these two ports means that there is more likelihood of picking up contacts that reside in, or have visited areas close to, these airports.
The introduction of the new airports caused the IPS research team to review the way that traffic from airports not sampled by the IPS is accounted for in the IPS processing systems. As a result, the systems were modified slightly in order to prevent overestimates or underestimates of traffic occurring at a regional level.
The introduction of the new airports and the subsequent changes made to the processing systems causes a discontinuity in the IPS results. Any comparisons of IPS results for 2005 onwards with earlier years (and especially those of a UK regional nature) therefore should be made with care.
Changes in methodology, 2007
The methods of computing expenditure (imputation) for cases where no expenditure information is given by the contact changed in 2007. The new method takes account of the duration of stay of the contact which had not been the case previously and means there may be a discontinuity in the expenditure series from 2006 to 2007. The new methodology compensates for possible overestimates of spending which may have arisen in the past due to the average daily spending being generally lower on longer trips than on shorter ones.
The costs of a package trip normally include fares to and from the country of visit. For expenditure estimates the fares are deducted from the cost of a package in order to obtain the amount of spending on the visit. The manual method of looking up fares from brochures and from the web was replaced in 2007 by an automated system which uses fares data provided by the respondent.
Coding of UK towns
In 2007 a more comprehensive approach to coding UK towns was introduced. Interviewers were provided with a more detailed list of towns and boroughs than in the past, meaning that their recording of responses given by respondents was more accurate.
Changes in methodology, 2009
Aberdeen Airport was introduced to the sample, and as a result, the estimated number of visits to cities and regions in Scotland will have been impacted positively. Belfast International Airport was also introduced but visits to cities and regions in Northern Ireland are not reported in the IPS Overseas Travel and Tourism estimates due to inability to record details of visits made by crossing the Irish land border.
Prior to 2009, known passenger traffic passing through Belfast was allocated to airports in Great Britain. The allocation of this traffic to interviews conducted in Belfast in 2009 will have had some downward impact on estimates of visits to towns and regions in Great Britain. (Airports at Doncaster, Southampton and Bournemouth were added in 2008)
More broadly, the overall methodology of the IPS was changed in 2009, in terms of both sampling and data processing.
Sampling was revised to incorporate an increase in the number of shifts run at many ports outside of Heathrow and a decrease in the number of shifts run at Heathrow. This change was introduced following a Port Survey Review (65 Kb Pdf) in response to the recommendations put forward by the Inter-Departmental Task Force on Migration Statistics.
Further, the way that shifts are run was changed via the introduction of a system employing a primary sampling interval for screening migrants and a sub-sample interval for travel and tourism contacts. This approach didn’t affect the profile of travel and tourism contacts but it did require a change in the way the data is processed.
The data processing involves weighting of all records and imputation of records with information missing at certain questions. The basic principles behind the processing were retained in 2009 but improvements were made in some aspects. This resulted in some discontinuity (35.2 Kb Pdf) with a downward impact of approx 2% in visits to the UK and 3% in visits overseas and a further value of less than 1% in earnings and expenditure.
There have been no changes in data collection methodology since 2009. However, the methodology used to estimate the number of UK residents departing from or arriving at UK ports on cruises was revised in 2010. The new methodology utilises new sources of data, including that published by DfT, IRN Research and the European Cruise Council. This represents an improvement in methodology and has the effect of increasing the estimated number of visits to ‘rest of the world’ by UK residents by approximately 175,000 compared with 2009.
Notes to tables and figures
The following conventions have been used in the tables:
0 denotes a figure of less than 0.5
. indicates that data are not available
The sum of spending across sub-categories of visit may not add to total spending. Spend per visit and spend per day by overseas visitors broken down by some categories of visit cannot be calculated by dividing spending by the number of visits. See Appendix B for details. In some cases, percentages in tables in this report from years prior to 2004 may differ by 1.0% from those published in previous years. This is because of changes in the method of rounding figures. The figures in this report are the most accurate.
The IPS is a large continuous survey and ONS would not be able to carry out the survey without the efforts of many different groups of people from a variety of organisations. In particular, ONS wishes to acknowledge the parts played by the following:
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The interviewers for their role in collecting the information on which the results of the IPS are based.
The respondents for the information they have provided.
The operators and managers of seaports, airports and rail terminals who give IPS interviewers access to their facilities in order to interview passengers.
The companies and organisations that provide additional information and data which enable the IPS results to be produced.