National Travel Survey 2022: Quality report

Q: About this release

This document supports the National Travel Survey ( NTS ) statistics. The NTS is a household survey of personal travel by residents of England travelling within Great Britain, from data collected via interviews and a one week travel diary. The NTS is part of a continuous survey that began in 1988, following ad-hoc surveys from the 1960s, which enables analysis of patterns and trends.

Question 1

National Travel Survey 2022: Quality Report

Accepted Answer

About this release

This document supports the National Travel Survey (NTS) statistics. The NTS is a household survey of personal travel by residents of England travelling within Great Britain, from data collected via interviews and a one week travel diary. The NTS is part of a continuous survey that began in 1988, following ad-hoc surveys from the 1960s, which enables analysis of patterns and trends.

About the National Travel Survey

The National Travel Survey (NTS) is a diary-based travel survey designed to provide a rich source of data on personal travel and has been running since the mid-1960s. The survey is primarily designed to track long-term development of trends; therefore, care should be taken when drawing conclusions from short-term changes.

NTS data is collected on behalf of DfT by our contractor, the National Centre for Social Research (NatCen), via two main methods: a household survey conducted face-to-face with all members of the household, followed by a 7-day travel diary for each household member. The NTS covers travel by people in all age groups, including children, across England.

Impact of the Covid-19 pandemic

Since March 2020, NTS data collection has been affected by varying restrictions associated with the coronavirus (COVID-19) pandemic. Following a pause in fieldwork in March and April 2020, the NTS has since relied upon data collection via ‘push-to-telephone’ with interviews being conducted over the telephone and interviewers completing the travel diary on behalf of respondents. During 2021, a ‘knock-to-nudge’ approach was introduced, whereby an interviewer would knock at an address’s door to encourage participation (but not enter the address) and arrange to complete the interview and diaries via telephone.

Due to the emergence of the Omicron variant in late 2021, the knock-to-nudge approach was retained for the first quarter of 2022. As the effects of the pandemic eased, the NTS returned to face-to-face interviewing from April onwards, retaining the option of a telephone backup.

Over the course of the pandemic, the field force of interviewers available to work on the NTS reduced substantially, meaning that in 2022 not all households could be surveyed face-to-face as planned, instead relying on the push-to-telephone method employed as a back-up. Across the whole of 2022, this method was used for some 1,666 addresses, or 13% of the full sample (12,852 households.) Response rates for push-to-telephone were substantially lower, meaning that this is one of the key reasons for the overall 2022 response rate being lower than in pre-pandemic years. The push-to-telephone method was not retained for 2023.

Analysis of the responding sample shows that the profile of respondents was more similar to those achieved in pre-pandemic years, however some differences remain. Along with a more even response rate across the year, and a lower proportion of responses collected by telephone, this means that a lower level of corrective weighting was needed, and therefore the effective sample size for 2022 was larger than in 2021 and the data considered more robust, despite a smaller number of responses.

More details on the changes made to fieldwork operations and weighting strategies in 2020 to 2022, and their impact on the data, can be found Chapter 1 of the Technical Report.

Changes to the 2022 survey

In 2022, two new modes of transport were added to the list, ferries and mobility scooters, following the introduction of codes for e-scooters and e-bikes in 2021. Previously, trips and trip stages using e-scooters and mobility scooters were coded at data entry stage as ‘other – private transport’, ferries were ‘other – public transport’, and e-bikes were coded as ‘pedal cycle’. DfT has reviewed the data gathered in 2021 and 2022 for these new mode codes. A low number of trips using these modes was recorded, however there are few data sources against which to validate this data. DfT will keep data for the new modes under review with the intention of presenting these separately when we have confidence in the quality of this data. For more details, see section 5.11 of the 2023 Technical Report.

A large-scale test of a new digital travel diary will begin in January 2024. Within the digital diary, the modes of transport will be presented to respondents in a radio button list, in contrast to the paper diary which has only a blank box. We will test to see whether this causes a change in rates of recording of trips, including the new modes, and this will help to validate existing data.

Quality of the NTS data

The NTS is produced to high professional standards set out in the Code of Practice for Statistics. The NTS was confirmed as National Statistics in July 2011 by the UK Statistics Authority and passed its most recent Office for Statistics Regulation compliance check in September 2018.

Strengths and limitations of the NTS

The NTS is a world leading travel survey and many other countries have used the methodologies as a base for their own travel surveys. The longevity, quality of data collected and stringent quality controls in place mean it is widely considered as the ‘gold standard’ of travel surveys. This section highlights the key strengths and limitations of the NTS.

Strengths

Long term and continuous: The NTS first ran in 1965 and has been running on an annual basis since 1988. The methodology has been broadly unchanged meaning that long-term trends can be monitored, which is a key purpose of the NTS

Detailed travel pattern data collected: The NTS collects a rich level of travel data including who, how, why and when people travelled. There is no other data source which collects this level of detail at a national level.

Large and representative sample: A lot of work is put in to obtain a suitable sample size that is representative of England’s population. This makes it possible to analyse data by various demographics such as age, sex, region and ethnic group.

Inclusivity: As a household survey with a randomly stratified sample, every address in England has a known and calculable chance of being invited to participate in the survey. Provisions are made to include those who may not have a sufficient level of English language capability to participate, by matching those households to interviewers who can speak alternative languages. The diary is normally completed by pen and paper, however since the start of the covid-19 pandemic, diaries have been completed by interviewers remotely when they have been unable to visit households in person. Whilst the Covid-19 restrictions have undoubtedly harmed response rates, the telephone method has allowed those in the vulnerable ‘shielding’ groups and others with concerns to participate.

Following a successful period of development, a new digital travel diary is being tested ahead of its potential introduction as a data collection tool in 2025. Further details on this work are available

Strong, established reputation: The NTS is widely considered the gold-standard of travel surveys and produces statistics to a high quality standard. Consequently the NTS is used to inform the evidence base for many different transport policies in the Department for Transport as well as being used by many external organisations including transport planners and academics.

Open data: The NTS dataset is freely accessible from the UK Data Service for users who wish to explore micro-level data for themselves.

User engagement: To ensure that our statistics continue to meet the needs of our users, the NTS team runs user engagement exercises when changes to the survey or published statistics are proposed, both internally within the Department for Transport and externally via our webpages

Limitations

Lower level geographies: The NTS is not designed to produce robust data below regional level. Whilst it is possible to analyse data for smaller geographies than regions, for example local authorities, often many years of data need to be combined to obtain a suitable sample size. Even then this is not ideal as weightings are applied to the sample to be representative of England. This is likely to skew analyses as demographics at sub-national level can vary significantly from the national level.

Difficult to perform multiple breakdowns: Just as with analyses for smaller geographies, it is also difficult to obtain a large enough sample size to produce robust analysis for specific groups which require multiple demographic breakdowns (for example analysing motorcycle trips of men over the age of 50 in London).

Self-reporting may not reflect actual travel behaviour: Disadvantages to relying on self-reporting include inaccurate recall, forgetting to write journeys down and wrongly estimating time and distances of journey (for example rounding a 7 minute journey to 10 minutes). Whilst there are extensive validation checks in place to minimise error, it is not possible to eliminate them entirely.

Limitations to the amount of data that can be collected: Whilst the NTS collects a rich level of detail of travel patterns, it is not currently able to collect other types of data such as journey satisfaction or how participants would prefer to travel (for example a participant may have taken the bus but would have preferred to travel by train if that travel option was available). The NTS seeks to achieve a balance between achieving its key aims and avoiding over-burdening respondents. Since 2019, DfT has run the National Travel Attitudes Study which captures topical information from previous NTS respondents, such as attitudes towards cycling and towards travelling using various methods during the pandemic, and this serves to address some of these gaps.

Not fully inclusive: Even though a household survey is considered one of the most inclusive methodologies for surveys, the NTS does exclude certain groups such as people with no fixed abode or people living in communal establishments such as residential care homes.

Limited geographical coverage: The NTS used to cover all households in Great Britain but since 2013 it has covered England only. The reasons for this are outlined in the 2011 Consultation on the Future Design of the National Travel Survey.

Covid-19 impact The pandemic and its associated restrictions and uncertainty has resulted in changes to the operation of the survey fieldwork, and as a consequence the survey has achieved a lower response rate and therefore reduced statistical and analytical power, especially in 2020 and 2021 , and to a lesser extent in 2022.

Sampling

Sample selection

The NTS is designed to provide a representative sample of households in England and is based on a stratified, clustered random sample of 12,852 private households. This sampling frame is the Postcode Address File (PAF), which is a list of all addresses in England. Postcode sectors are employed as Primary Sampling Units (PSUs). The sample is drawn by selecting 756 PSUs and then by selecting 17 addresses within each PSU. The NTS uses a quasi-panel design, where half the PSUs in a given year’s sample are retained for the next year’s sample and the other half are replaced. This has the effect of reducing the variance of estimates of year-on-year change. For example 378 of the PSUs selected for the 2017 sample were retained for the 2018 sample, supplemented with 378 new PSUs.

In 2020 the monthly sample was doubled from August onwards to counter the lower response rate achieved in the push-to-telephone mode. This gave an overall sample of 17,136 private households for 2020. The sample reverted to its normal size in 2021 with the introduction of the ‘knock-to-nudge’ method and remained the same in 2022.

Stratification

Grouped postcode sectors in England are stratified using a regional variable, an urban or rural indicator, car ownership and a working from home indicator, all drawn from Census data. This is done to increase the precision of the sample and to ensure that the different strata in the population are correctly represented.

Ineligible households

There are some address types which are classified as ineligible to participate in the NTS. These types of addresses include houses which are not yet built or under construction, vacant houses and non-residential addresses such as an address occupied solely by a business.

London

Response rates tend to be much lower in London compared with the rest of England. The NTS oversamples London with the aim of achieving responding sample sizes in London and elsewhere which are proportional to their population.

Start date

Since 2014 interviewers have been assigned to start on different dates across the month to ensure that the interviewing and travel week start dates are evenly spread across the month (Chart 1). This reduces sample bias and means there is more data available for analysis on days of the month which were previously under-represented.

In 2020, due to the move to push-to-telephone interviewing, there was a switch to a rolling travel week. In the face-to-face design a fixed travel week approach is used where households are given a specific week in which to record their travel. This can sometimes be two weeks or more after their initial interview. Given the restriction around face-to-face contact, and the increased risk of drop-of at the diary completion stage, the 2020 survey moved to a rolling travel week approach, which was maintained during 2021. Under the rolling travel week approach the travel week always started the day before the date of the interview. This was to allow completion of the first day of the travel diary during the initial interview. Upon return to face-to-face in quarter 2 2022, fixed travel weeks were reintroduced, resulting in a more even spread of data across the days of the week.

Chart 1 and 2: Percentage of travel weeks which start on each day of the month, England, 2020 and 2022

Response rates

Only data for fully co-operating households are included in the final diary dataset used for analysis. There are 2 ways of measuring response rates: the achieved sample rate and the standard response rate. The achieved response rate is the percentage of fully co-operating households amongst all addresses selected in the sampling frame. The standard response rate is the percentage of fully co-operating households amongst the eligible households within the sampling frame. Ineligible households such as empty homes are excluded from the total; usually about 10% of selected households are ineligible to participate in the NTS.

Definitions

Fully cooperated: All household members fully completed the interview and travel diary.

Partially cooperated: All household members completed the interview but not all completed the travel diary.

Chart 3: NTS achieved sampling rate and standard response rate: England, 2022

Standard response rates declined to 53% in 2017 before increasing slightly to 54% in 2019. Due to changes in data collection due to the coronavirus (COVID-19) pandemic in 2020 the response rate dropped to 16%. In 2021 the response rate partially recovered to 38% despite ongoing pandemic-related restrictions, largely due to the introduction of the knock-to-nudge methodology. A reduced field force in 2022, along with a slightly higher rate of refusals and an increase in the proportion of partially productive cases, contributed to a reduction in the response rate for this year to 31%.

**Chart 4: Standard response rate: England 2013 to 2022 **

Quality control measures

As National Statistics, NTS statistics are produced to high professional standards set out in the Code of Practice for statistics and many quality control measures are in place to ensure the integrity of the data.

Sampling: As detailed in the sampling section, the NTS is designed to provide a representative sample of households in England and the sampling methodology has been refined and improved over the years. Weights are applied to the sample to reduce non-response bias.

Start date: It is important that the choice of Travel Week and its start date is not left to the discretion of the respondent or interviewer as this could lead to bias. To prevent bias, diary travel weeks are evenly spread over the days of the week as well as the weeks of the quota month. If the respondent says they are unable to begin recording their Travel Week on the assigned start date then they are removed from the survey.

The allocation of travel weeks was affected during the pandemic period, with rolling travel weeks utilised from 2020 to quarter 1 2022. With the reintroduction of face-to-face interviewing, fixed travel weeks were also reintroduced. Interviewer standards: Interviewers play a crucial role in the delivery of the NTS. All interviewers recruited to work on the NTS have significant experience working on other major surveys. Interviewers new to the NTS receive a 2 day briefing which covers all aspects of the survey and includes role-play exercises to practice. They receive a comprehensive set of instructions which they can refer to throughout fieldwork, and all new interviewers are accompanied by an experienced interviewer on their first day working on the NTS. Interviewers also attend a one day refresher briefing every year to be trained on any changes being made to the survey. Interviewers are set clear assignment-level performance targets which include a deadline for completion, coverage milestones, a requirement for all cases to be contacted within the first seven days of the wave, and minimum expected response rate. During the fieldwork period, close attention is paid to response rates and coverage so that swift action can be taken to remedy any potential shortfall. There are a number of other performance indicators that are to be monitored regularly at an assignment level, such as number of completed interviews, hours worked, strike rate achieved, number of broken appointments and number of refusals.

Interviews are back-checked to ensure that interviewers were working to the standards to which they were trained and in accordance with survey requirements. A minimum of 10% of the total productive interviews are back-checked, the majority (usually 90%) by telephone but by letter where this was not possible. If the responses received indicate significant deviations from the standards set, a supervisor will revisit the address(es) concerned personally. Most back-checking is carried out within 2 weeks, and always within 4 weeks, of the interview date. Back-checking has found no systematic errors in the way interviewers are working. All interviewers are also subject to twice yearly supervisions to confirm that they are working to the highest standards.

Mid-week check: Interviewers are encouraged to check on respondents halfway through the Travel Week in order to encourage and help out respondents with any difficulties they might be experiencing whilst filling out their travel diaries. This could be either a phone call or a personal visit and is at the interviewer’s discretion, although they are strongly encouraged to conduct a face-to-face check for elderly participants. During the operation of the push-to-telephone method from 2020, mid-week checks have continued in the form of a telephone call, during which the interviewers collect several days’ worth of travel diary entries. In 2022 74% of fully productive households had a mid-week check, compared with 44% in 2020 and 80% in 2019. See technical report chapter 4

Incentivising respondents: Incentives are offered to participants in order to maximise response rates, although as mentioned these have been falling in recent years. All households invited to participate are given a book of 6 First Class stamps when sent the invitation letter, and households may keep these regardless of whether they decide to participate or not. If the household participates in the interview and all household members complete the travel diary, then everybody in the household is given a £5 gift voucher. Text message reminders are also sent to households (who agree to be contacted in this way) to remind them when the Travel Week is beginning to reduce the chances of households forgetting to complete the diary. We are currently undertaking research investigating how to improve response rates, including recent experiments exploring the effects of increasing the incentive values and sending out a new advance letter inviting participation to the survey. Reports for the research so far have been published.

During 2020 a temporary change was made to the incentive strategy, following the relaxation of rules around proxy interviews, and the expected fall in the response rate, a £20 household-level incentive was given, regardless of the number in the household. In 2021 and 2022 this continued for telephone respondents, whereas the usual £5-per-respondent was reintroduced for knock-to-nudge households and subsequent face-to-face households.

Gazetteer: The NTS uses a Gazetteer of over 100,000 places in Great Britain to check the starting places of trips, and their destinations, and the distance between them. During the interview and the data checking stage, the CAPI and Diary Entry System uses the gazetteer’s grid references to calculate reasonably precise distances between each named location using checks based on straight line distances. For trips of 15 miles or over, respondents’ estimates of distance are flagged for checking if they are not between 0.75 and 1.75 as the crow fly miles at the data processing stage. Discrepancies in distance estimates are not flagged where respondent and crow fly miles are both below 15 miles. Alongside this, trip times are also checked with respect to the distance. For example, if the respondent says a 100 mile car trip took 20 minutes then that is impossible and would be corrected. If a 2 mile car trip took an hour, then that is unlikely but not impossible as the respondent could have been stuck in stationery traffic. If the distance and time were correct this would be noted on the diary by the interviewer so it would not be rechecked by the coder later on.

Pick-up interview: After the end of the Travel Week the interviewer will conduct a short interview known as the pick-up interview. The two main purposes of this is to collect vehicle mileage information and also to check if there have been any changes since the household interview. For example, the pick-up interview checks if any vehicles have been acquired or disposed of and whether any new driving licences or season tickets have been acquired since the initial interview.

Validation checks: There are validation checks on the data at all stages of processing. The first stage of checking is done by interviewer at the mid-week interview to check that the respondent is filling out the diary correctly, and then by the interviewer at the pick-up interview where they will check each diary to make sure that all the necessary information has been included. When converting the diaries into a dataset, data coders will contact interviewers for clarification on any diary data that is unclear (for example if they are unable to read handwriting) or seems unusual (such as no return trip entered). In some cases the interviewer will go back to the respondent for clarification. There are many further quality assurance checks in place both when NatCen compile and clean the dataset and when the DfT NTS team produce the statistics based on the underlying dataset.

Survey review: Every year the NTS is reviewed to assess if any questions need to be added, removed, or amended to ensure the survey meets user needs as much as possible. Any new questions undergo in-depth levels of pilot testing to check that they are fit for purpose and understood by respondents. Recent examples include the request for user feedback on proposal for changes to NTS questions and the cognitive testing of new questions.

Imputation

There is a relatively small number of variables which undertake an imputation process where missing values are derived by looking at other known data. A variety of techniques are used in the imputation routines and are automated and run in a specific order due to the dependencies between variables. In 2017, 49 out of 935 variables contained imputed data. Of these 49 variables, 34 had less than 1% of their cases imputed. Table 1 lists the 16 variables which had more than 0.5% of their cases imputed in 2022.

Table 1: NTS variables with more than 0.5% of cases imputed, 2022

Variable	% of imputed cases
Household:HHIncome_Imp	39.7
Individual:IndIncome_Imp	17.9
Stage:NumBoardings_Imp	91.3
Stage:StageTime_Imp	0.7
Stage:StageVehicle_Imp	0.6
Trip:TripTotalTime_Imp	1.3
Trip:TripTravTime_Imp	0.6
Vehicle:EngineCap_Imp	9.8
Vehicle:RegLetter_Imp	8.7
Vehicle:RegYear_Imp	3.0
Vehicle:VehAge_Imp	2.8
Vehicle:VehAnMileage_Imp	5.2
Vehicle:VehBusMile_Imp	24.9
Vehicle:VehComMile_Imp	25.3
Vehicle:VehPriMile_Imp	27.0
Vehicle:VehRank_Imp	5.2

There are some variables with noticeably high imputation rates, although none are considered as a cause for concern and there are legitimate reasons for them. For example, the ‘NumBoardings’ variable records the number of boardings for a stage of a public transport journey and has an imputation rate of 91.3%. Whilst this may seem high, for almost all these cases the number of boardings does not actually apply (for example, when travelling by private modes of transport) and so a value of ‘0’ is imputed into the final dataset. Further details about these variables can be found on the NTS Documentation section of the UK Data Archive.

Real household income equivalence

To allow analysis of trip behaviour by income on a comparable basis, households are categorised into income bands based on a measure of household affluence known as real household income equivalence. This adjusts a household’s stated income so that the households size and composition are taken into account. This adjustment is carried out using a measure called the McClements Scale. Incomes are also adjusted for inflation to facilitate analysis across time periods.

Completing the survey by proxy

The NTS covers people of all ages and a consequence of this is a significant amount of data is collected by proxy (Chart 5), that is that someone in the household completes the survey on behalf of another household member (or members). For example the individual questionnaire is not asked to children under 11 years old and is completed on their behalf by an adult in the household. There are also instances where not all adults in the household are present and so the interview can be completed by another adult on their behalf. It is also common for the travel diary to be completed by proxy for similar reasons in that there are children which would be too young to complete it (although unlike the interview, there is no set age limit) or that it is not always practical for every adult in the house to complete it directly themselves. During 2020 and 2021, to facilitate interviewing by telephone, rules on proxy interviews were relaxed. Rules around completion of travel diaries were also changed, moving responsibility for diary completion from respondents to interviewers. Previous protocols were restored on return to face-to-face.

Chart 5: Proportion of interviews completed directly or by proxy, by age: England, 2022

Data revisions

The NTS does not have any regular scheduled data revisions and the statistics published each year are usually considered as final. There have been 2 major revisions made to NTS data in the last 15 years. These revisions are the result of methodological improvements made which could be applied to previous years in some form. These improvements were applying weightings to the NTS sample and changing the diary day when short walk data is collected.

Details of these changes can be found in the 2020 quality report.

Standard errors

Prior to 2002, standard errors were calculated for the NTS every 3-4 years. Since 2005 the NTS has applied a weighting strategy to compensate for non-response bias. In 2010, the Office for National Statistics (ONS) designed a methodology for calculating standard errors with a weighted NTS sample and applied this to the 2009 dataset. The methodology and set of standard errors are available from the NTS standard errors guide web page. The process for calculating standard errors in this way however was resource intensive and so it was not possible to produce an update on standard errors on NTS data for the years after this.

The process for producing these has now been implemented into R (as opposed to STATA, although the underlying methodology remains the same) which has led to efficiency improvements which will allow us to update these tables more frequently in the future. We produced standard errors for some key tables for the 2018 dataset. For comparative purposes, Table 2 presents 2009 and 2018 standard error data for trips per person per year by main mode. It should be noted that 2009 covers Great Britain and 2018 covers England, and the 2009 table presents the standard errors for the unrevised short walk figures (see page 12). However the standard errors and level of confidence for the statistics are broadly similar to each other.

Table 2: Standard errors of trips per person per year by mode: 2009 (GB)

Main mode	Mean (trips)	Standard Error	95% confidence interval lower	95% confidence interval upper	Deft
Walk	227.7	4.7	218.5	237.0	1.6
Bicycle	15.3	0.8	13.7	16.9	1.4
Car/van driver	394.6	4.5	385.8	403.4	1.2
Car/van passenger	217.1	2.7	211.8	222.5	1.2
Motorcycle	3.0	0.3	2.4	3.7	1.2
Other private transport	8.9	0.7	7.5	10.3	1.6
Bus in London	18.7	1.4	16.0	21.5	1.9
Other local bus	48.6	1.6	45.4	51.8	1.7
Non-local bus	0.6	0.1	0.4	0.8	1.4
London underground	9.0	0.9	7.2	10.8	2.0
Surface rail	16.3	0.8	14.8	17.8	1.4
Taxi/minicab	11.1	0.5	10.2	12.0	1.4
Other public transport	2.0	0.4	1.3	2.7	2.1
All modes	973.0	6.9	959.4	986.6	1.6

Table 3: Standard errors of trips per person per year by mode: 2018 (England)

Main mode	Mean (trips)	Standard Error	95% confidence interval lower	95% confidence interval upper	Deft
Walk	262.5	5.8	251.1	273.8	1.5
Bicycle	17.1	1.1	14.9	19.2	1.4
Car/van driver	395.0	5.0	385.1	404.8	1.3
Car/van passenger	207.5	3.2	201.1	213.8	1.4
Motorcycle	1.9	0.3	1.3	2.5	1.3
Other private transport	7.5	0.7	6.1	8.8	1.4
Bus in London	15.2	0.9	13.4	16.9	1.5
Other local bus	32.7	1.4	29.9	35.4	1.5
Non-local bus	0.4	0.1	0.3	0.5	1.2
London underground	10.8	1.0	8.9	12.7	1.9
Surface rail	22.1	1.2	19.7	24.6	1.8
Taxi/minicab	10.4	0.6	9.2	11.6	1.6
Other public transport	3.4	0.6	2.2	4.7	2.1
All modes	986.3	8.1	970.4	1002.2	1.6

Standard errors

Sampling error in any survey arises because the variable estimates are based on a sample rather than a full census of the population. The results obtained for any single sample varies slightly from the true values for the population. The difference between the estimates derived from the sample and the true population values is referred to as the standard error. Standard errors can help provide guidance when attempting to establish the level of confidence of the statistics produced. Large samples within the NTS often have a high degree of confidence but there will be lower degrees of confidence for smaller samples (for example cycling trip rates amongst males aged over 80 in London).

R and its survey package was used to produce the standard errors. For further information see:

R Core Team (2018). R: A language and environment for statistical computing. R Foundation for Statistical Computing, Vienna, Austria. R project

T. Lumley (2017) “survey: analysis of complex survey samples”. R package version 3.32.

T. Lumley (2004) Analysis of complex survey samples. Journal of Statistical Software 9(1): 1-19

Data confidentiality and security

Confidentiality

Respondents are informed at the beginning of the survey process in the advance letter about data confidentiality and that participation is voluntary. The Frequently Asked Questions that accompany the advance letter outline the NTS compliance with the current General Data Protection Regulations. The NTS is collected for reasons of public interest. Information provided by respondents is confidential and is not passed on to anyone outside NatCen or the statistics section at DfT in a form that could be used to identify them. Respondents are provided with a telephone number for NatCen’s Operations Department to contact if they have any queries.

Security

Interviewers and remote coders (who conduct data entry for the travel diary) have a dedicated work laptop and data is transferred directly via NatCen via a secure internet collection. These laptops are PGP encrypted and have secure account logins (managed via Microsoft Active Directory) and require password changes every 30 days. The servers the data is uploaded to has high security measures in place and access is restricted to the relevant teams. Development is carried out in accordance with ISO27001 for Information and Security Management.

Methodological changes

One of the NTS’ key strengths is continuity over time, however the NTS team, in conjunction with NatCen, keeps aspects of the methodology and survey fieldwork operation under review. A webpage detailing the results of various projects is available.

The key methodological improvements made to the NTS since it began over 50 years ago are:

1965 First National Travel Survey
1972 to 73 Travel Diary start date made more flexible
1978 to 79 Travel of children under 3 included for the first time
1988 Continuous surveying introduced
1994 Computer Assisted Personal Interviewing (CAPI) introduced
2002 Sample size increased to 16,000 individuals; vehicle registration data collected to link to Driver and Vehicle Licencing Agency records; coding transferred from interviewers to central coding team
2003 Conditional incentives introduced
2004 Unconditional incentive of a book of stamps introduced
2005 Data weighted for the first time
2006 Imputation indicators added; long distance journeys asked only for previous week, improving accuracy of recall
2007 New place names gazetteer introduced
2013 Reduced clustering of addresses leads to an improvement in sampling efficiency
2014 Change to the way start weeks were allocated, providing a more even spread of data
2016 Attitudinal questions asked of a randomly selected household member rather than the head of household, making data more representative
2017 Short walks recorded on Day 1 of the travel diary for the full sample for the first time
2018 Experiments with different levels of incentives and different advance letters; introduction of the National Travel Attitudes Study

Refusal questionnaire

In 2018 a ‘refusal’ or ‘non-response’ questionnaire was introduced to capture basic household level information for households which chose not to take part in the survey. This consisted of three questions covering the number of adults in the household, the tenure of the household and whether anyone in the household owns or has continuous use of any motor vehicles. Findings from this questionnaire are planned to be incorporated within a wider review of the NTS weighting over the next few years.

The National Travel Survey results are available in publications, factsheets and data tables.

Full guidance on the methods used to conduct the survey, response rates, weighting methodology and survey materials can be found in the National Travel Survey Technical Report.

A notes and definitions document which includes background to the NTS, response rates, sample size and standard error information plus a full list of definitions.

Accessing micro-level NTS data for analysis

In addition to the published statistics described in this document together with accompanying statistical tables, the underlying dataset and guidance in analysing it can be accessed from the UK Data Service or the Office for National Statistics Secure Research Service for users who wish to explore the data for themselves.

To hear more about DfT statistics publications as they are released please follow us on Twitter via DfTstats.

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Further information

National Travel Survey statistics

Email national.travelsurvey@dft.gov.uk

Public enquiries 020 7944 3077

Media enquiries 0300 7777 878

Question 2

About this release

Accepted Answer

This document supports the National Travel Survey (NTS) statistics. The NTS is a household survey of personal travel by residents of England travelling within Great Britain, from data collected via interviews and a one week travel diary. The NTS is part of a continuous survey that began in 1988, following ad-hoc surveys from the 1960s, which enables analysis of patterns and trends.

Question 3

Impact of the Covid-19 pandemic

Accepted Answer

Since March 2020, NTS data collection has been affected by varying restrictions associated with the coronavirus (COVID-19) pandemic. Following a pause in fieldwork in March and April 2020, the NTS has since relied upon data collection via ‘push-to-telephone’ with interviews being conducted over the telephone and interviewers completing the travel diary on behalf of respondents. During 2021, a ‘knock-to-nudge’ approach was introduced, whereby an interviewer would knock at an address’s door to encourage participation (but not enter the address) and arrange to complete the interview and diaries via telephone.

Due to the emergence of the Omicron variant in late 2021, the knock-to-nudge approach was retained for the first quarter of 2022. As the effects of the pandemic eased, the NTS returned to face-to-face interviewing from April onwards, retaining the option of a telephone backup.

Over the course of the pandemic, the field force of interviewers available to work on the NTS reduced substantially, meaning that in 2022 not all households could be surveyed face-to-face as planned, instead relying on the push-to-telephone method employed as a back-up. Across the whole of 2022, this method was used for some 1,666 addresses, or 13% of the full sample (12,852 households.) Response rates for push-to-telephone were substantially lower, meaning that this is one of the key reasons for the overall 2022 response rate being lower than in pre-pandemic years. The push-to-telephone method was not retained for 2023.

Analysis of the responding sample shows that the profile of respondents was more similar to those achieved in pre-pandemic years, however some differences remain. Along with a more even response rate across the year, and a lower proportion of responses collected by telephone, this means that a lower level of corrective weighting was needed, and therefore the effective sample size for 2022 was larger than in 2021 and the data considered more robust, despite a smaller number of responses.

More details on the changes made to fieldwork operations and weighting strategies in 2020 to 2022, and their impact on the data, can be found Chapter 1 of the Technical Report.

Question 4

Changes to the 2022 survey

Accepted Answer

In 2022, two new modes of transport were added to the list, ferries and mobility scooters, following the introduction of codes for e-scooters and e-bikes in 2021. Previously, trips and trip stages using e-scooters and mobility scooters were coded at data entry stage as ‘other – private transport’, ferries were ‘other – public transport’, and e-bikes were coded as ‘pedal cycle’. DfT has reviewed the data gathered in 2021 and 2022 for these new mode codes. A low number of trips using these modes was recorded, however there are few data sources against which to validate this data. DfT will keep data for the new modes under review with the intention of presenting these separately when we have confidence in the quality of this data. For more details, see section 5.11 of the 2023 Technical Report.

A large-scale test of a new digital travel diary will begin in January 2024. Within the digital diary, the modes of transport will be presented to respondents in a radio button list, in contrast to the paper diary which has only a blank box. We will test to see whether this causes a change in rates of recording of trips, including the new modes, and this will help to validate existing data.

Quality of the NTS data

The NTS is produced to high professional standards set out in the Code of Practice for Statistics. The NTS was confirmed as National Statistics in July 2011 by the UK Statistics Authority and passed its most recent Office for Statistics Regulation compliance check in September 2018.

Question 5

Strengths and limitations of the NTS

Accepted Answer

The NTS is a world leading travel survey and many other countries have used the methodologies as a base for their own travel surveys. The longevity, quality of data collected and stringent quality controls in place mean it is widely considered as the ‘gold standard’ of travel surveys. This section highlights the key strengths and limitations of the NTS.

Strengths

Long term and continuous: The NTS first ran in 1965 and has been running on an annual basis since 1988. The methodology has been broadly unchanged meaning that long-term trends can be monitored, which is a key purpose of the NTS

Detailed travel pattern data collected: The NTS collects a rich level of travel data including who, how, why and when people travelled. There is no other data source which collects this level of detail at a national level.

Large and representative sample: A lot of work is put in to obtain a suitable sample size that is representative of England’s population. This makes it possible to analyse data by various demographics such as age, sex, region and ethnic group.

Inclusivity: As a household survey with a randomly stratified sample, every address in England has a known and calculable chance of being invited to participate in the survey. Provisions are made to include those who may not have a sufficient level of English language capability to participate, by matching those households to interviewers who can speak alternative languages. The diary is normally completed by pen and paper, however since the start of the covid-19 pandemic, diaries have been completed by interviewers remotely when they have been unable to visit households in person. Whilst the Covid-19 restrictions have undoubtedly harmed response rates, the telephone method has allowed those in the vulnerable ‘shielding’ groups and others with concerns to participate.

Following a successful period of development, a new digital travel diary is being tested ahead of its potential introduction as a data collection tool in 2025. Further details on this work are available

Strong, established reputation: The NTS is widely considered the gold-standard of travel surveys and produces statistics to a high quality standard. Consequently the NTS is used to inform the evidence base for many different transport policies in the Department for Transport as well as being used by many external organisations including transport planners and academics.

Open data: The NTS dataset is freely accessible from the UK Data Service for users who wish to explore micro-level data for themselves.

User engagement: To ensure that our statistics continue to meet the needs of our users, the NTS team runs user engagement exercises when changes to the survey or published statistics are proposed, both internally within the Department for Transport and externally via our webpages

Limitations

Lower level geographies: The NTS is not designed to produce robust data below regional level. Whilst it is possible to analyse data for smaller geographies than regions, for example local authorities, often many years of data need to be combined to obtain a suitable sample size. Even then this is not ideal as weightings are applied to the sample to be representative of England. This is likely to skew analyses as demographics at sub-national level can vary significantly from the national level.

Difficult to perform multiple breakdowns: Just as with analyses for smaller geographies, it is also difficult to obtain a large enough sample size to produce robust analysis for specific groups which require multiple demographic breakdowns (for example analysing motorcycle trips of men over the age of 50 in London).

Self-reporting may not reflect actual travel behaviour: Disadvantages to relying on self-reporting include inaccurate recall, forgetting to write journeys down and wrongly estimating time and distances of journey (for example rounding a 7 minute journey to 10 minutes). Whilst there are extensive validation checks in place to minimise error, it is not possible to eliminate them entirely.

Limitations to the amount of data that can be collected: Whilst the NTS collects a rich level of detail of travel patterns, it is not currently able to collect other types of data such as journey satisfaction or how participants would prefer to travel (for example a participant may have taken the bus but would have preferred to travel by train if that travel option was available). The NTS seeks to achieve a balance between achieving its key aims and avoiding over-burdening respondents. Since 2019, DfT has run the National Travel Attitudes Study which captures topical information from previous NTS respondents, such as attitudes towards cycling and towards travelling using various methods during the pandemic, and this serves to address some of these gaps.

Not fully inclusive: Even though a household survey is considered one of the most inclusive methodologies for surveys, the NTS does exclude certain groups such as people with no fixed abode or people living in communal establishments such as residential care homes.

Limited geographical coverage: The NTS used to cover all households in Great Britain but since 2013 it has covered England only. The reasons for this are outlined in the 2011 Consultation on the Future Design of the National Travel Survey.

Covid-19 impact The pandemic and its associated restrictions and uncertainty has resulted in changes to the operation of the survey fieldwork, and as a consequence the survey has achieved a lower response rate and therefore reduced statistical and analytical power, especially in 2020 and 2021 , and to a lesser extent in 2022.

Question 6

Sampling

Accepted Answer

Sample selection

The NTS is designed to provide a representative sample of households in England and is based on a stratified, clustered random sample of 12,852 private households. This sampling frame is the Postcode Address File (PAF), which is a list of all addresses in England. Postcode sectors are employed as Primary Sampling Units (PSUs). The sample is drawn by selecting 756 PSUs and then by selecting 17 addresses within each PSU. The NTS uses a quasi-panel design, where half the PSUs in a given year’s sample are retained for the next year’s sample and the other half are replaced. This has the effect of reducing the variance of estimates of year-on-year change. For example 378 of the PSUs selected for the 2017 sample were retained for the 2018 sample, supplemented with 378 new PSUs.

In 2020 the monthly sample was doubled from August onwards to counter the lower response rate achieved in the push-to-telephone mode. This gave an overall sample of 17,136 private households for 2020. The sample reverted to its normal size in 2021 with the introduction of the ‘knock-to-nudge’ method and remained the same in 2022.

Stratification

Grouped postcode sectors in England are stratified using a regional variable, an urban or rural indicator, car ownership and a working from home indicator, all drawn from Census data. This is done to increase the precision of the sample and to ensure that the different strata in the population are correctly represented.

Ineligible households

There are some address types which are classified as ineligible to participate in the NTS. These types of addresses include houses which are not yet built or under construction, vacant houses and non-residential addresses such as an address occupied solely by a business.

London

Response rates tend to be much lower in London compared with the rest of England. The NTS oversamples London with the aim of achieving responding sample sizes in London and elsewhere which are proportional to their population.

Start date

Since 2014 interviewers have been assigned to start on different dates across the month to ensure that the interviewing and travel week start dates are evenly spread across the month (Chart 1). This reduces sample bias and means there is more data available for analysis on days of the month which were previously under-represented.

In 2020, due to the move to push-to-telephone interviewing, there was a switch to a rolling travel week. In the face-to-face design a fixed travel week approach is used where households are given a specific week in which to record their travel. This can sometimes be two weeks or more after their initial interview. Given the restriction around face-to-face contact, and the increased risk of drop-of at the diary completion stage, the 2020 survey moved to a rolling travel week approach, which was maintained during 2021. Under the rolling travel week approach the travel week always started the day before the date of the interview. This was to allow completion of the first day of the travel diary during the initial interview. Upon return to face-to-face in quarter 2 2022, fixed travel weeks were reintroduced, resulting in a more even spread of data across the days of the week.

Chart 1 and 2: Percentage of travel weeks which start on each day of the month, England, 2020 and 2022

Question 7

Response rates

Accepted Answer

Only data for fully co-operating households are included in the final diary dataset used for analysis. There are 2 ways of measuring response rates: the achieved sample rate and the standard response rate. The achieved response rate is the percentage of fully co-operating households amongst all addresses selected in the sampling frame. The standard response rate is the percentage of fully co-operating households amongst the eligible households within the sampling frame. Ineligible households such as empty homes are excluded from the total; usually about 10% of selected households are ineligible to participate in the NTS.

Definitions

Fully cooperated: All household members fully completed the interview and travel diary.

Partially cooperated: All household members completed the interview but not all completed the travel diary.

Chart 3: NTS achieved sampling rate and standard response rate: England, 2022

Standard response rates declined to 53% in 2017 before increasing slightly to 54% in 2019. Due to changes in data collection due to the coronavirus (COVID-19) pandemic in 2020 the response rate dropped to 16%. In 2021 the response rate partially recovered to 38% despite ongoing pandemic-related restrictions, largely due to the introduction of the knock-to-nudge methodology. A reduced field force in 2022, along with a slightly higher rate of refusals and an increase in the proportion of partially productive cases, contributed to a reduction in the response rate for this year to 31%.

**Chart 4: Standard response rate: England 2013 to 2022 **

Question 8

Quality control measures

Accepted Answer

As National Statistics, NTS statistics are produced to high professional standards set out in the Code of Practice for statistics and many quality control measures are in place to ensure the integrity of the data.

Sampling: As detailed in the sampling section, the NTS is designed to provide a representative sample of households in England and the sampling methodology has been refined and improved over the years. Weights are applied to the sample to reduce non-response bias.

Start date: It is important that the choice of Travel Week and its start date is not left to the discretion of the respondent or interviewer as this could lead to bias. To prevent bias, diary travel weeks are evenly spread over the days of the week as well as the weeks of the quota month. If the respondent says they are unable to begin recording their Travel Week on the assigned start date then they are removed from the survey.

The allocation of travel weeks was affected during the pandemic period, with rolling travel weeks utilised from 2020 to quarter 1 2022. With the reintroduction of face-to-face interviewing, fixed travel weeks were also reintroduced. Interviewer standards: Interviewers play a crucial role in the delivery of the NTS. All interviewers recruited to work on the NTS have significant experience working on other major surveys. Interviewers new to the NTS receive a 2 day briefing which covers all aspects of the survey and includes role-play exercises to practice. They receive a comprehensive set of instructions which they can refer to throughout fieldwork, and all new interviewers are accompanied by an experienced interviewer on their first day working on the NTS. Interviewers also attend a one day refresher briefing every year to be trained on any changes being made to the survey. Interviewers are set clear assignment-level performance targets which include a deadline for completion, coverage milestones, a requirement for all cases to be contacted within the first seven days of the wave, and minimum expected response rate. During the fieldwork period, close attention is paid to response rates and coverage so that swift action can be taken to remedy any potential shortfall. There are a number of other performance indicators that are to be monitored regularly at an assignment level, such as number of completed interviews, hours worked, strike rate achieved, number of broken appointments and number of refusals.

Interviews are back-checked to ensure that interviewers were working to the standards to which they were trained and in accordance with survey requirements. A minimum of 10% of the total productive interviews are back-checked, the majority (usually 90%) by telephone but by letter where this was not possible. If the responses received indicate significant deviations from the standards set, a supervisor will revisit the address(es) concerned personally. Most back-checking is carried out within 2 weeks, and always within 4 weeks, of the interview date. Back-checking has found no systematic errors in the way interviewers are working. All interviewers are also subject to twice yearly supervisions to confirm that they are working to the highest standards.

Mid-week check: Interviewers are encouraged to check on respondents halfway through the Travel Week in order to encourage and help out respondents with any difficulties they might be experiencing whilst filling out their travel diaries. This could be either a phone call or a personal visit and is at the interviewer’s discretion, although they are strongly encouraged to conduct a face-to-face check for elderly participants. During the operation of the push-to-telephone method from 2020, mid-week checks have continued in the form of a telephone call, during which the interviewers collect several days’ worth of travel diary entries. In 2022 74% of fully productive households had a mid-week check, compared with 44% in 2020 and 80% in 2019. See technical report chapter 4

Incentivising respondents: Incentives are offered to participants in order to maximise response rates, although as mentioned these have been falling in recent years. All households invited to participate are given a book of 6 First Class stamps when sent the invitation letter, and households may keep these regardless of whether they decide to participate or not. If the household participates in the interview and all household members complete the travel diary, then everybody in the household is given a £5 gift voucher. Text message reminders are also sent to households (who agree to be contacted in this way) to remind them when the Travel Week is beginning to reduce the chances of households forgetting to complete the diary. We are currently undertaking research investigating how to improve response rates, including recent experiments exploring the effects of increasing the incentive values and sending out a new advance letter inviting participation to the survey. Reports for the research so far have been published.

During 2020 a temporary change was made to the incentive strategy, following the relaxation of rules around proxy interviews, and the expected fall in the response rate, a £20 household-level incentive was given, regardless of the number in the household. In 2021 and 2022 this continued for telephone respondents, whereas the usual £5-per-respondent was reintroduced for knock-to-nudge households and subsequent face-to-face households.

Gazetteer: The NTS uses a Gazetteer of over 100,000 places in Great Britain to check the starting places of trips, and their destinations, and the distance between them. During the interview and the data checking stage, the CAPI and Diary Entry System uses the gazetteer’s grid references to calculate reasonably precise distances between each named location using checks based on straight line distances. For trips of 15 miles or over, respondents’ estimates of distance are flagged for checking if they are not between 0.75 and 1.75 as the crow fly miles at the data processing stage. Discrepancies in distance estimates are not flagged where respondent and crow fly miles are both below 15 miles. Alongside this, trip times are also checked with respect to the distance. For example, if the respondent says a 100 mile car trip took 20 minutes then that is impossible and would be corrected. If a 2 mile car trip took an hour, then that is unlikely but not impossible as the respondent could have been stuck in stationery traffic. If the distance and time were correct this would be noted on the diary by the interviewer so it would not be rechecked by the coder later on.

Pick-up interview: After the end of the Travel Week the interviewer will conduct a short interview known as the pick-up interview. The two main purposes of this is to collect vehicle mileage information and also to check if there have been any changes since the household interview. For example, the pick-up interview checks if any vehicles have been acquired or disposed of and whether any new driving licences or season tickets have been acquired since the initial interview.

Validation checks: There are validation checks on the data at all stages of processing. The first stage of checking is done by interviewer at the mid-week interview to check that the respondent is filling out the diary correctly, and then by the interviewer at the pick-up interview where they will check each diary to make sure that all the necessary information has been included. When converting the diaries into a dataset, data coders will contact interviewers for clarification on any diary data that is unclear (for example if they are unable to read handwriting) or seems unusual (such as no return trip entered). In some cases the interviewer will go back to the respondent for clarification. There are many further quality assurance checks in place both when NatCen compile and clean the dataset and when the DfT NTS team produce the statistics based on the underlying dataset.

Survey review: Every year the NTS is reviewed to assess if any questions need to be added, removed, or amended to ensure the survey meets user needs as much as possible. Any new questions undergo in-depth levels of pilot testing to check that they are fit for purpose and understood by respondents. Recent examples include the request for user feedback on proposal for changes to NTS questions and the cognitive testing of new questions.

Question 9

Imputation

Accepted Answer

There is a relatively small number of variables which undertake an imputation process where missing values are derived by looking at other known data. A variety of techniques are used in the imputation routines and are automated and run in a specific order due to the dependencies between variables. In 2017, 49 out of 935 variables contained imputed data. Of these 49 variables, 34 had less than 1% of their cases imputed. Table 1 lists the 16 variables which had more than 0.5% of their cases imputed in 2022.

Table 1: NTS variables with more than 0.5% of cases imputed, 2022

Variable	% of imputed cases
Household:HHIncome_Imp	39.7
Individual:IndIncome_Imp	17.9
Stage:NumBoardings_Imp	91.3
Stage:StageTime_Imp	0.7
Stage:StageVehicle_Imp	0.6
Trip:TripTotalTime_Imp	1.3
Trip:TripTravTime_Imp	0.6
Vehicle:EngineCap_Imp	9.8
Vehicle:RegLetter_Imp	8.7
Vehicle:RegYear_Imp	3.0
Vehicle:VehAge_Imp	2.8
Vehicle:VehAnMileage_Imp	5.2
Vehicle:VehBusMile_Imp	24.9
Vehicle:VehComMile_Imp	25.3
Vehicle:VehPriMile_Imp	27.0
Vehicle:VehRank_Imp	5.2

There are some variables with noticeably high imputation rates, although none are considered as a cause for concern and there are legitimate reasons for them. For example, the ‘NumBoardings’ variable records the number of boardings for a stage of a public transport journey and has an imputation rate of 91.3%. Whilst this may seem high, for almost all these cases the number of boardings does not actually apply (for example, when travelling by private modes of transport) and so a value of ‘0’ is imputed into the final dataset. Further details about these variables can be found on the NTS Documentation section of the UK Data Archive.

Question 10

Real household income equivalence

Accepted Answer

To allow analysis of trip behaviour by income on a comparable basis, households are categorised into income bands based on a measure of household affluence known as real household income equivalence. This adjusts a household’s stated income so that the households size and composition are taken into account. This adjustment is carried out using a measure called the McClements Scale. Incomes are also adjusted for inflation to facilitate analysis across time periods.

Question 11

Completing the survey by proxy

Accepted Answer

The NTS covers people of all ages and a consequence of this is a significant amount of data is collected by proxy (Chart 5), that is that someone in the household completes the survey on behalf of another household member (or members). For example the individual questionnaire is not asked to children under 11 years old and is completed on their behalf by an adult in the household. There are also instances where not all adults in the household are present and so the interview can be completed by another adult on their behalf. It is also common for the travel diary to be completed by proxy for similar reasons in that there are children which would be too young to complete it (although unlike the interview, there is no set age limit) or that it is not always practical for every adult in the house to complete it directly themselves. During 2020 and 2021, to facilitate interviewing by telephone, rules on proxy interviews were relaxed. Rules around completion of travel diaries were also changed, moving responsibility for diary completion from respondents to interviewers. Previous protocols were restored on return to face-to-face.

Chart 5: Proportion of interviews completed directly or by proxy, by age: England, 2022

Question 12

Data revisions

Accepted Answer

The NTS does not have any regular scheduled data revisions and the statistics published each year are usually considered as final. There have been 2 major revisions made to NTS data in the last 15 years. These revisions are the result of methodological improvements made which could be applied to previous years in some form. These improvements were applying weightings to the NTS sample and changing the diary day when short walk data is collected.

Details of these changes can be found in the 2020 quality report.

Question 13

Standard errors

Accepted Answer

Prior to 2002, standard errors were calculated for the NTS every 3-4 years. Since 2005 the NTS has applied a weighting strategy to compensate for non-response bias. In 2010, the Office for National Statistics (ONS) designed a methodology for calculating standard errors with a weighted NTS sample and applied this to the 2009 dataset. The methodology and set of standard errors are available from the NTS standard errors guide web page. The process for calculating standard errors in this way however was resource intensive and so it was not possible to produce an update on standard errors on NTS data for the years after this.

The process for producing these has now been implemented into R (as opposed to STATA, although the underlying methodology remains the same) which has led to efficiency improvements which will allow us to update these tables more frequently in the future. We produced standard errors for some key tables for the 2018 dataset. For comparative purposes, Table 2 presents 2009 and 2018 standard error data for trips per person per year by main mode. It should be noted that 2009 covers Great Britain and 2018 covers England, and the 2009 table presents the standard errors for the unrevised short walk figures (see page 12). However the standard errors and level of confidence for the statistics are broadly similar to each other.

Table 2: Standard errors of trips per person per year by mode: 2009 (GB)

Main mode	Mean (trips)	Standard Error	95% confidence interval lower	95% confidence interval upper	Deft
Walk	227.7	4.7	218.5	237.0	1.6
Bicycle	15.3	0.8	13.7	16.9	1.4
Car/van driver	394.6	4.5	385.8	403.4	1.2
Car/van passenger	217.1	2.7	211.8	222.5	1.2
Motorcycle	3.0	0.3	2.4	3.7	1.2
Other private transport	8.9	0.7	7.5	10.3	1.6
Bus in London	18.7	1.4	16.0	21.5	1.9
Other local bus	48.6	1.6	45.4	51.8	1.7
Non-local bus	0.6	0.1	0.4	0.8	1.4
London underground	9.0	0.9	7.2	10.8	2.0
Surface rail	16.3	0.8	14.8	17.8	1.4
Taxi/minicab	11.1	0.5	10.2	12.0	1.4
Other public transport	2.0	0.4	1.3	2.7	2.1
All modes	973.0	6.9	959.4	986.6	1.6

Table 3: Standard errors of trips per person per year by mode: 2018 (England)

Main mode	Mean (trips)	Standard Error	95% confidence interval lower	95% confidence interval upper	Deft
Walk	262.5	5.8	251.1	273.8	1.5
Bicycle	17.1	1.1	14.9	19.2	1.4
Car/van driver	395.0	5.0	385.1	404.8	1.3
Car/van passenger	207.5	3.2	201.1	213.8	1.4
Motorcycle	1.9	0.3	1.3	2.5	1.3
Other private transport	7.5	0.7	6.1	8.8	1.4
Bus in London	15.2	0.9	13.4	16.9	1.5
Other local bus	32.7	1.4	29.9	35.4	1.5
Non-local bus	0.4	0.1	0.3	0.5	1.2
London underground	10.8	1.0	8.9	12.7	1.9
Surface rail	22.1	1.2	19.7	24.6	1.8
Taxi/minicab	10.4	0.6	9.2	11.6	1.6
Other public transport	3.4	0.6	2.2	4.7	2.1
All modes	986.3	8.1	970.4	1002.2	1.6

Standard errors

Sampling error in any survey arises because the variable estimates are based on a sample rather than a full census of the population. The results obtained for any single sample varies slightly from the true values for the population. The difference between the estimates derived from the sample and the true population values is referred to as the standard error. Standard errors can help provide guidance when attempting to establish the level of confidence of the statistics produced. Large samples within the NTS often have a high degree of confidence but there will be lower degrees of confidence for smaller samples (for example cycling trip rates amongst males aged over 80 in London).

R and its survey package was used to produce the standard errors. For further information see:

R Core Team (2018). R: A language and environment for statistical computing. R Foundation for Statistical Computing, Vienna, Austria. R project

T. Lumley (2017) “survey: analysis of complex survey samples”. R package version 3.32.

T. Lumley (2004) Analysis of complex survey samples. Journal of Statistical Software 9(1): 1-19

Question 14

Data confidentiality and security

Accepted Answer

Confidentiality

Respondents are informed at the beginning of the survey process in the advance letter about data confidentiality and that participation is voluntary. The Frequently Asked Questions that accompany the advance letter outline the NTS compliance with the current General Data Protection Regulations. The NTS is collected for reasons of public interest. Information provided by respondents is confidential and is not passed on to anyone outside NatCen or the statistics section at DfT in a form that could be used to identify them. Respondents are provided with a telephone number for NatCen’s Operations Department to contact if they have any queries.

Security

Interviewers and remote coders (who conduct data entry for the travel diary) have a dedicated work laptop and data is transferred directly via NatCen via a secure internet collection. These laptops are PGP encrypted and have secure account logins (managed via Microsoft Active Directory) and require password changes every 30 days. The servers the data is uploaded to has high security measures in place and access is restricted to the relevant teams. Development is carried out in accordance with ISO27001 for Information and Security Management.

Question 15

Methodological changes

Accepted Answer

One of the NTS’ key strengths is continuity over time, however the NTS team, in conjunction with NatCen, keeps aspects of the methodology and survey fieldwork operation under review. A webpage detailing the results of various projects is available.

The key methodological improvements made to the NTS since it began over 50 years ago are:

1965 First National Travel Survey
1972 to 73 Travel Diary start date made more flexible
1978 to 79 Travel of children under 3 included for the first time
1988 Continuous surveying introduced
1994 Computer Assisted Personal Interviewing (CAPI) introduced
2002 Sample size increased to 16,000 individuals; vehicle registration data collected to link to Driver and Vehicle Licencing Agency records; coding transferred from interviewers to central coding team
2003 Conditional incentives introduced
2004 Unconditional incentive of a book of stamps introduced
2005 Data weighted for the first time
2006 Imputation indicators added; long distance journeys asked only for previous week, improving accuracy of recall
2007 New place names gazetteer introduced
2013 Reduced clustering of addresses leads to an improvement in sampling efficiency
2014 Change to the way start weeks were allocated, providing a more even spread of data
2016 Attitudinal questions asked of a randomly selected household member rather than the head of household, making data more representative
2017 Short walks recorded on Day 1 of the travel diary for the full sample for the first time
2018 Experiments with different levels of incentives and different advance letters; introduction of the National Travel Attitudes Study

Refusal questionnaire

In 2018 a ‘refusal’ or ‘non-response’ questionnaire was introduced to capture basic household level information for households which chose not to take part in the survey. This consisted of three questions covering the number of adults in the household, the tenure of the household and whether anyone in the household owns or has continuous use of any motor vehicles. Findings from this questionnaire are planned to be incorporated within a wider review of the NTS weighting over the next few years.

Question 16

Further information

National Travel Survey statistics

Email national.travelsurvey@dft.gov.uk

Public enquiries 020 7944 3077

Media enquiries 0300 7777 878

National Travel Survey 2022: Quality report

Applies to England