Accredited official statistics

Sea passenger statistics: Background quality report

Updated 11 December 2024

This background quality report relates to the publication of sea passenger statistics.

The purpose of this document is to provide users of the statistics with information about the quality of the outputs, measured against different dimensions of statistical quality.

As a result, this document helps to demonstrate how the Department for Transport (DfT) complies with the Code of Practice for Official Statistics principle on quality.

These statistics were independently reviewed by the Office for Statistics Regulation in February 2013 and are therefore labelled ‘accredited official statistics’. Accredited official statistics are produced to high professional standards as set out in the Code of Practice for Statistics. They undergo regular quality assurance to ensure they meet customer needs.

Code of Practice for Statistics

The Code of Practice sets out common standards that should be followed by all UK organisations that produce official Code of Practice statistics.

Section 1: Background to the statistics

These statistics cover international and domestic passengers, with data collected and published differently for different types of route. They are collected for a range of uses.

International short sea ferry passenger statistics are collected monthly from ferry operators. They include drivers of lorries, coaches and other vehicles. Passenger figures for domestic ferry routes from Great Britain to Northern Ireland, the Channel Islands and the Isle of Man are also collected but not published monthly.

Cruise passenger figures include all passengers on cruise journeys who start and finish their cruise journey at a UK port as well as cruises between a UK port and a European or a Mediterranean port. Cruise passenger numbers are included in our statistics only for passengers at UK ports, whether they are departing, arriving or both. Cruise figures are compiled on an annual basis. Long sea voyage passengers are those travelling on one-way scheduled voyages to and from ports outside Europe or the Mediterranean. Passengers travelling on cargo vessels (one-way) are not collected.

In 2021, passenger numbers on UK domestic cruises were collected for the first time. Large domestic cruises did not previously occur but were a feature in 2021 due to the start of domestic cruising which was carried out as part of the gradual lifting of coronavirus (COVID-19) restrictions when the Foreign Office advised against international sea-going cruise travel due to public health medical advice. Domestic cruise passenger figures include all passengers on domestic cruises in the UK (no cruise stops at foreign ports).

Domestic ferry passenger figures on sea crossings to Orkney and Shetland are provided on an annual basis by the Scottish Government. Figures for some other sea crossings and river ferries are also provided annually by the operators.

Sea passenger statistical publications are published in 2 parts. Total international short sea passenger numbers are published monthly. The annual publication is published in August each year and includes all international and domestic passengers for which data are collected.

For further information, including a breakdown of the routes covered, see the notes and definitions document.

Methodology and production

Sea passenger data is collected using surveys, which are sent to operators as email attachments. The process is outlined below.

 The monthly collection covers passengers on short sea routes with UK ports on vessels over 100 gross tonnes. A form with cover letter and instructions are sent to operators.

The annual domestic passenger collection data (as with the monthly collection) is restricted to vessels over 100 gross tonnes. The collection includes river ferries in the UK that have a distance greater than 500 metres and passenger kilometres greater than 500.

The annual cruise collection includes cruise ships which return to the port of departure as well as those which let passengers off at a destination port. The operators label the type of cruise as “Europe”, “world” or “liner”. Europe and world cruises are classed as “cruise” in the data. Liner cruises are classed as “long sea” in the data. The definitions of each cruise are:

• Europe (E) - cruise in Europe or the Mediterranean (including one-way trip)
• world (W) - cruise outside Europe or the Mediterranean (excluding one-way trips)
• liner (L) - voyage (one-way voyage to or from a port outside Europe and the Mediterranean)

For each monthly and annual collection process, data returns are collated together using R (a statistical programming software).

DfT validates the data to verify that the data items are accurate and consistent. This includes checking for duplicates, missing data and any anomalies, particularly flagging figures that are either considerably high or low compared to the previous month or year. These checks are done using R.

Any anomalies are investigated and where we believe they may be inaccurate, queries are sent back to the operator for confirmation of the data. DfT then signs off the data for use in production of the statistics.

Once the data is finalised, it is entered into a SQL/GCP database, where data tables are exported to Excel and the tables and statistical release are produced for publication. Further checks are then carried out, for example looking at trends over time.

Each statistical release produced is independently checked by a statistician where any discrepancies are resolved before being approved by the senior statistician.

Section 2: Quality assessment

In this section, the quality of the statistics is considered in relation to the different dimensions of quality as stated in the European Statistical System (ESS) quality framework.

European Statistical System (ESS) quality framework:

The European Statistical System (ESS) handbook for quality reports provides comprehensive guidance on measuring the quality of statistical processes.

  1. Relevance

Relevance is the degree to which a statistical product meets user needs in terms of content and coverage.

The statistical outputs presented within sea passenger statistics include:

• a statistical release containing key findings, trends over time and signposts to further information.
• Open Document Spreadsheet (ODS) data tables containing information on the number passengers by port and ferry route
• a dashboard that displays interactive visualisation of international sea passenger volumes by journey type, UK port and destinations
• a notes and definitions document outlining route level breakdown and strengths and weaknesses

Known users and uses of the statistics

Within the DfT the statistics are used:

• for ministerial briefing and to answer public enquiries
• as background to policy development, for example as an evidence base for cyber security policy decisions
• for monitoring trends in sea passenger activity
• by analysts in modelling overall passenger trends, or for ad-hoc work for example related to assessing the scale and impact of disruption to key routes

Outside of DfT, known users include:

• operators of sea passenger services, port authorities and any other maritime organisations monitoring sea passenger activity
• the Office for National Statistics use monthly data for grossing the international passenger survey and in their statistical releases
• MET police, who occasionally use the data to help inform policing of ports operation
• Home Office, who use data in preparing their accredited official statistics release (immigration statistics)
• other more general users who want to gain an overview of the sea passenger business, and for benchmarking

How well the statistics meet user needs

DfT carried out an internal review of the sea passenger statistics in 2017 to assess if the statistics are compliant with the code of practice. The review covered things like, timeliness of the publication, explored the need for other potential data sources and checked to see if the statistical series is meeting user needs.

While the conclusion was that the statistics are generally fit for the purposes for which they are used, a number of requests were noted. These are summarised below, along with the department’s response.

User requests	Outcome
A breakdown of the figures for domestic passengers by individual route.	The department has checked with operators whether they would be content for a more detailed breakdown by route to be published. However, the data at this level are considered commercially sensitive, and data suppliers did not want DfT to publish further data on individual routes.
The number of cars, coaches and lorries carried, as well as passengers.	Data on vehicles can be found in tables PORT0301 and PORT0205 of the Port Freight statistics published by DfT.
A distinction between foot passengers and passengers travelling by vehicle.	Whilst this data has been collected on an ad-hoc basis from some operators, we do not feel there is currently sufficient demand to increase the burden on operators.
Operators’ forecast of passengers and vehicles carried.	These forecasts are commercially sensitive, therefore we are unable to include them in the publication.
Monthly data for domestic short sea passengers.	Due to the way domestic data are collected (monthly and annual), we will maintain the current approach to aggregate it annually unless there are further user requests for monthly data.
Increase coverage of the statistics to include other services (for example, those using vessels under 100 gross tonnes).	DfT considers that the added value of seeking to collect data for these services would not justify the increased burden of data collection, which would fall on smaller operators. The consistency of the published time series would also be affected.

  2. Accuracy and reliability

Accuracy refers to how close the estimated value in the output is to the true result.

These statistics aim to measure the true number of sea passengers travelling to and from UK ports. Overall, high level comparisons with industry sources (where available) and trends over time suggest that the statistics are likely to provide a consistent approximation overall, though it is possible that in some areas (for example, cruise passengers and domestic services) figures are slightly under-estimated.

The following summarises potential sources of risks or errors which may arise throughout the process of compiling the sea passenger statistics.

Potential source of error	Risk of error mitigation
Lloyds Registration Number (LRN) errors - invalid vessels	LRN is a unique vessel identifier. The most common error with LRN is that the contributor will have made a typo. There are validation checks in place which flags when the LRN number is greater than or less than 7 numbers. The validation also flags if it does not recognise the LRN. DfT then will rectify all the flagged LRN’s.
Number of sailing errors	If there are big discrepancies between the number of sailings arriving and departing, DfT will query with the operators to confirm the data.
Duplicate entries	Duplicate entries are identified, confirmed with operators and deleted.
Unexpected routes	Unexpected routes are flagged and queried with an operator to check if the new route is accurate.
Passenger numbers	Passenger figures by routes are compared to the same month previous year. Figures are investigated if they exceed or are below the 20% benchmark.
Missing data	On rare occasions, when the DfT get no response from operators, we estimate the data. The estimation is based on an average of previous records.
Port locode errors	Locodes are port identifiers. Typos and incorrect locodes are picked up by validation routines run before the data is processed into the database.
New operator or route	We are continuously on the lookout for new operators or routes (for example monitoring trade press), to avoid underestimating passenger numbers wherever possible.

Revision of 2023 and 2021 cruise and long sea passenger data

Following the discovery of some duplicate figures in our raw data affecting the number of passengers that travelled on cruise and long sea journeys, revisions were made to the 2023 and 2021 cruise and long sea passenger figures in the latest Sea passenger statistics.

The revision made to the cruise and long sea passenger figures has had a minor effect on the total number of sea passengers for 2023 and 2021. This change has not affected the overall trends. Tables SPAS0101, SPAS0107 and SPAS0201 and the web release have been updated with the revised figures and charts.

DfT are in regular contact with the data providers. This ensures a good working relationship and timely responses when querying issues with the operators. DfT on a monthly and annual basis carry out validations to ensure the data is of sufficient quality. Although it is unusual that the statistics will be revised at a later stage, we want to ensure the data remains reliable even after it has been published.

The revision of 2023 and 2021 figures involved the presence of duplicate records that were not detected during validation. To reduce the chances of this happening again, DfT has taken some further steps to improve our validation process in regard to this source of error.

Firstly, addressing the instance of duplication at the time of collection, by improving the collection method with data providers, so duplicates are less likely to occur. DfT has also introduced a further check that flags records for additional review. Detailed instructions have been written to guide through the new process.

Scale of change

As mentioned above, the revision has had a minor effect on sea passenger figures for 2023 and 2021. For 2023, the percentage change has been a 4% decrease in the revised international cruise passenger figures compared to pre-revision numbers, a 10% decrease in the revised long sea passenger figures and a 10% decrease in the revised domestic cruise passenger figures. For 2021, the revised international cruise figures were 3% lower than pre revision and domestic cruise figures were 7% lower than pre revision, while long sea passenger figures were unaffected.

  3. Timeliness and punctuality

Timeliness describes the time between the date of publication and the date to which the data refers, and punctuality describes the time between the actual publication and the planned publication of a statistic.

Sea passenger statistics are currently published monthly (SPAS0107) and  annually (all other tables) for international, cruise and domestic passengers are published in August). The production of the monthly table commences in the middle of the month once the data has been collected. The table is finalised and published 6 weeks after the reference month. Cruise and domestic data is collected from January to April, with the data cleaned and validated in May and June. The annual release is started at the end of June and published in August.

To date, all sea passenger statistical releases have been published to the scheduled pre-announced date.

  4. Accessibility and clarity

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

Accessibility

The outputs are published on the GOV.UK DfT statistics page in accessible formats.

Statistical releases are available as html files, which are accessible for those who use assistive technologies.

Data tables are available in ODS file format which can be accessed by using freely available software. The spreadsheets have been designed to be used easily by assistive technologies. The published tables include information on the passenger numbers by port, ferry route and country of destination.

Clarity

The statistical releases use plain language, in which technical terms, acronyms and definitions are defined where appropriate. The main findings are presented using a series of text, charts and maps, with maps used to show the location of the major ports. Full details of the strengths and weaknesses of the data are provided at the end of each statistical report for users who are interested in this. In addition to the statistical releases, a notes and definitions document has been published which lists the routes the sea passenger series covers as well as the key definitions.

  5. Coherence and comparability

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

Comparability

The statistical series for international sea passengers covers trends since 1950 for all short sea routes. The data are considered to be comparable over time, showing an increase in sea passenger journeys up to 1994 (when the Channel Tunnel opened). Since then, there has been a slow decline in sea passenger numbers.

Long sea journeys are also considered broadly comparable over time. The trends for long sea journeys during the 1950s to the 1970s show liner journeys were popular for inter-continental travel. Since then, there has been a decrease, likely to be due to the increase in demand for air travel. Numbers can fluctuate due to the use of journeys to reposition vessels by some operators.

Comparison with figures produced by industry bodies suggests that the trend shown for cruise journeys are reliable; however, a port survey carried out in early 2013 resulted in more cruise data becoming available for Harwich, Newcastle and Portsmouth, which may impact on the trends shown for these ports (at the overall level the impact is likely to be minor, compared to the real increase in demand for cruises).

A larger operator returned figures in 2019 that hadn’t been provided previously, with vessels operating at the port of Dover. This contributed to an increase in cruise passengers at this port.

Furlough of port employees for Aberdeen and Inverness due to the COVID-19 pandemic meant that figures for these ports could not be included in 2019. Both reporting issues mentioned were assessed to be small compared to the total and did not impact the overall trends for cruises and long sea passengers.

The domestic sea passenger movements are on the whole considered to reflect trends over time, however figures can be affected by changes in operator recording practices (for example part of the increase in river ferry passengers in the 2017 statistics is attributed to a change in recording by a large operator).

Coherence

For cruises, Cruise Lines International Association (CLIA) also publishes statistics on UK resident passengers. CLIA figures do not count the same thing as they are counting UK residents on cruises ships across the world, and therefore the numbers do not match the DfT source. However, trends across the 2 data sources generally align when there are any increases or decreases in passenger numbers.

For other types of sea passengers, fewer sources are available to make comparisons.

6. Trade-offs between output quality components

Trade offs between output quality components describes the extent to which different aspects of quality are balanced against each other.

Previously, there were 2 annual statistical publications due to cruise and domestic data taking longer to collect. DfT have engaged with operators and have been able to bring the collection timescales forward. This means the data is ready to be published in August each year. As the data is ready earlier, the 2 publications have been combined into one release to improve the overall timeliness of our statistics for users.

 7. Assessment of user needs and perceptions

Assessment of user needs and perception covers the processes for finding out about users and uses, and their views on the statistical products.

DfT regularly engages with users by social media, email and face to face methods when possible. This includes requesting feedback on the monthly and annual statistics series, with contact details provided in every statistical release. Each publication is promoted via X (formerly known as Twitter).

DfT also regularly analyses web page usage, ad-hoc requests and social media analytics to monitor activity over time. In line with the Code of Practice for Statistics, users will be informed about any changes or revisions to the data series. Less frequently, the department reviews the statistics (last done in 2017) and presents key results at seminars, for example those arranged by the Transport Statistics User Group (last presentation on sea passenger statistics in March 2017). As part of the annual publication issued in November 2018, the department published a note to users listing changes made and proposals to the data tables that accompany the publication. This included informing known users about the planned changes and requesting feedback.

In December 2018, the department conducted a user feedback survey to understand if the statistics continue to be fit for purpose and meet user needs. The survey also asked for views on the actions detailed in the note to users the department published (see above). A summary of the feedback has been published detailing user responses to the survey and the department’s course of action as a result. One of these actions was to cease updating quarterly tables as, after assessment, the monthly table is sufficient to meet user needs.

8. Performance, cost and respondent burden

Performance, cost and respondent burden describes the effectiveness, efficiency and economy of the statistical output.

The overall respondent burden related to these statistics is considered to be relatively small. There are around 20 operators that supply data monthly, typically larger ferry operators, and it is considered that in most cases the figures can be easily extracted from their administrative or management information systems (with the data being recorded by the company for other purposes), so that the only additional burden relates to the compilation of the monthly return.

Cruise and long sea returns are provided monthly by some operators, and annually by others. Data returned consists of a list of passenger numbers by voyage which it is assumed are already captured in management information systems.

The domestic sea passenger returns are considered to be generally straightforward, with operators permitted to provide best estimates where data are not held.

In DfT, the estimated resource involved in the sea passenger statistics is assumed to be under 0.5 FTE (Full-Time Equivalent), which is considered to be proportionate.

9. Confidentiality, transparency and security

Confidentiality, transparency and security refers to the procedures and policy used to ensure sound confidentiality, security and transparent practices.

All data is stored, accessed and analysed using DfT secure IT systems. Data protection regulations are adhered to throughout the sea passenger statistics production process, and any information provided to DfT by operators are kept securely where access to data is controlled in accordance with departmental policy.

The information used to compile these statistics provides details on company name, company code, port names, vessel details and figures for number of passengers and sailings arriving and departing. Sensitive or personal passenger details are not collected. Passenger numbers by company are collected. However, the published statistics do not include passenger figures by individual company, as data is presented in an aggregated format. No statistical disclosure control methods are applied to the outputs, as the variables included in the statistics are not considered to be sensitive.

DfT aims to publish as much data as is possible whilst ensuring that confidentiality is maintained.

DfT adheres to the principles and protocols laid out in the Code of Practice for Statistics and comply with pre-release access arrangements. The pre-release access lists are available on the DfT website.

Section 3: Summary and conclusions

This background quality report presents information for users of the sea passengers statistics covering different aspects of their quality. The department concludes, on the basis of the assessment outline above, that the statistics are of a quality which is considered fit for the purposes for which the statistics are being used.

Comments and feedback on this report, or any other aspect of these statistics are welcome, and can be provided by email to sea-passenger.stats@dft.gov.uk.

Further information about these sea passenger statistics is available, including:

• data tables
• notes and definitions