Chapter 3 Sample Selection
3.1 Sample size and structure
The NTS 2025 was designed to provide a representative sample of households in England and was based on a stratified 2-stage random probability sample of private households. The sampling frame was the ‘small user’ Postcode Address File (PAF), a list of all addresses in the country (also known as delivery points).
The sample for the 2025 survey was drawn firstly by selecting the Primary Sampling Units (PSUs), and then by selecting addresses within PSUs. The sample design employs postcode sectors as PSUs. Each PSU represents one sample point (also known as an assignment), and for fieldwork purposes each point is issued to an individual interviewer.
For NTS 2025, the sample size was increased compared with NTS 2024. 1,440 PSUs were selected for the core sample and a further 888 for the reserve sample. 22 addresses were drawn from each PSU, equating to a total of 31,680 selected core addresses and 19,536 reserve addresses. However, not all the reserve sample was issued based on the lower response rates in early 2025, therefore issuing the full reserve was not considered necessary. Of the 888 PSUs in the reserve, 158 were randomly selected to issue. Consequently, an additional 3,476 addresses were issued in the latter half of the NTS 2025 fieldwork year.
3.2 Quasi-panel design
Following a review of the NTS methodology in 2000, it was decided that the NTS should introduce a quasi-panel design from 2002 onwards. According to this design, 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.
Therefore 582 of the PSUs selected for the 2024 sample were retained for the 2025 core sample. As the overall sample size increased for NTS 2025, these 582 retained PSUs were supplemented with 1,746 new PSUs (858 for the core sample and 888 for the reserve sample). The PSUs carried over from the 2024 sample for inclusion in 2025 were excluded from the 2025 sample frame, so they could not appear twice in the sample, however, the dropped PSUs from 2024 were included.
Whilst the same PSU postcode sectors might appear in different survey years, no single addresses were allowed to be included in 3 consecutive years to minimise the chances of the same address being selected again. Each year, NatCen provides the sampling company with a list of the addresses selected for the previous 3 survey years. These addresses were excluded from the sampling frame before the addresses for 2025 were selected. This means respondents to the 3 previous year’s surveys in the carried over PSUs could not be contacted again.
For further information about the methodological review, see Elliott, D. (2000) ONS Quality Review of the National Travel Survey: Some Aspects of Design and Estimation Methods.
3.3 Selection of sample points
Sample points were selected firstly by generating a list of all postcode sectors in England (excluding those in the Isles of Scilly due to cost of interviewing). Sectors carried over from the previous year were also excluded, as described in section 3.2 above. Sectors with fewer than 500 delivery points were grouped with an adjacent sector. Grouped sectors were then treated as one PSU. On average each PSU contained about 3,250 delivery points.
This list of grouped postcode sectors in England was then stratified using the 4 stratification variables agreed in the 2023 NTS Sampling Stratification Review. These consist of a regional variable, an urban or rural indicator, a car ownership indicator, and a commuting travel mode indicator. The 3 latter variables are derived from data collected during the 2021 Census. This was done to increase the precision of the sample and to ensure that the different strata in the population are correctly represented. Random samples of PSUs were then selected within each stratum.
The regional strata for England are based on the International Territorial Level 2 (ITL2) areas, formerly NUTS2, grouped in a few cases where single areas are too small. International Territorial Levels (formerly known as Nomenclature of Units for Territorial Statistics) replaces the European-wide geographical classification developed by the European Office for Statistics (Eurostat) following the UK withdrawal from the EU. The 2 classifications are equivalent. ITL2 roughly relates to counties or groups of counties in England. The 33 regional strata for the survey are shown in Table 3.1, along with the region codes that each of the strata belong to.
Table 3.1: NTS regional stratification variable
| Stratification number | England | Region code |
|---|---|---|
| 1 | Inner London – East | 8 Inner London |
| 2 | Inner London – West | 8 Inner London |
| 3 | Outer London – East and North East | 9 Outer London |
| 4 | Outer London – South | 9 Outer London |
| 5 | Outer London West and North West | 9 Outer London |
| 6 | Devon and Cornwall | 6 South West |
| 7 | Dorset and Somerset | 6 South West |
| 8 | Bath, Bristol, Gloucestershire and Wiltshire | 6 South West |
| 9 | Oxfordshire, Buckinghamshire and Berkshire | 10 South East |
| 10 | Hampshire and Isle of Wight | 10 South East |
| 11 | Kent | 10 South East |
| 12 | Surrey, West Sussex and East Sussex | 10 South East |
| 13 | Essex | 7 East of England |
| 14 | East Anglia | 7 East of England |
| 15 | Hertfordshire and Bedfordshire | 7 East of England |
| 16 | Leicestershire, Rutland and Northamptonshire | 4 East Midlands |
| 17 | Lincolnshire | 4 East Midlands |
| 18 | Warwickshire, Herefordshire and Worcestershire | 5 West Midlands |
| 19 | West Midlands | 5 West Midlands |
| 20 | Shropshire and Staffordshire | 5 West Midlands |
| 21 | Nottinghamshire and Derbyshire | 4 East Midlands |
| 22 | Cheshire | 2 North West |
| 23 | Merseyside | 2 North West |
| 24 | Greater Manchester | 2 North West |
| 25 | Lancashire and Cumbria | 2 North West |
| 26 | East Yorkshire and North Lincolnshire | 3 Yorkshire and the Humber |
| 27 | South Yorkshire | 3 Yorkshire and the Humber |
| 28 | West Yorkshire | 3 Yorkshire and the Humber |
| 29 | North Yorkshire | 3 Yorkshire and the Humber |
| 30 | Tees Valley and Durham | 1 North East |
| 31 | Northumberland and Tyne and Wear | 1 North East |
Within each region, postcode sectors were allocated to “urban” or “rural” based on the urban or rural indicator. The urban rural indicator itself was based on the 2021 Census and derived from the 10-category Rural Urban Classification. Within subcategory, postcode sectors were sorted by 2 further variables from the 2021 Census: the percentage of households with no car within small areas in tertiles then proportion of employed persons aged 16 or over travelling to work by car or van.
In the next step of the process, 1,746 postcode sectors were systematically selected for the core sample with probability proportional to delivery point count. Differential sampling fractions were used in Inner London, Outer London and the rest of England in order to oversample London (see section 3.4 for further details). These sectors were then added to the 582 sectors carried over from the previous year’s survey to produce the initial core sample of 1,440 sectors and a reserve sample of 888 sectors.
3.4 Oversampling of London
Each year, London PSUs are oversampled. Response rates tend to be much lower in London compared with the rest of England, with rates being lowest in Inner London. The NTS oversamples Inner and Outer London with the aim of achieving responding sample sizes in London and elsewhere which are proportional to their population. Estimates of response rates were made to oversample Inner and Outer London based on recent years of NTS. Of the 2,328 PSUs in the sample drawn, 225 were in Outer London and 166 in Inner London.
3.5 Self-completion section
Starting in NTS 2017, a Computer Assisted Self Interviewing (CASI) module for transport satisfaction questions was added, where one adult from those present during the household interview is asked to complete the satisfaction questions.
The CASI sample for NTS 2025 was recruited using an equal probability of selection, except in households where both people aged 16 to 29 and 30 or over were present. In such households, those aged 16 to 29 were selected with an 80% probability. This differential selection probability was then adjusted for in the weighting of the CASI responding sample.
3.6 Allocation of PSUs to months
To allocate core PSUs evenly across NTS 2025, the survey year was divided into 12 quota (fieldwork) months and equal numbers of PSUs (360) were initially assigned to each quarter, resulting in an average of 120 points being issued each month. All reserve PSUs were allocated to quarter 3 and 4, 444 in each quarter and an average of 148 per month.
Allocating PSUs evenly across a quarter (rather than a month) results in a more even spread of the average number of points and hence interviews and travel diaries per day across months. This approach makes it easier to control for variation across seasons. Furthermore, PSUs were allocated to quota months such that a nationally representative sample would be obtained for each quarter.
As noted in section 3.3 above, random samples of PSUs were selected within each stratum, as well as being evenly spread across each quarter. The distribution of core sample points for each quota month across the major regional strata is shown in Appendix L. Subsequently, 158 PSUs were selected from the 888 PSU reserve to be issued. Prior to random selection of the 158, the whole reserve was sorted by start date in order to preserve the random allocations.
3.7 Fieldwork start dates
Since 2014, an additional process followed the selection of sample points. As part of this process, start dates are evenly spread across each month and then assigned to the points per month at random to provide an even spread of responses across the year.
3.8 Selection of households at sampled addresses
Interviewers should interview only one household per address given to them in their sample point. At some addresses, interviewers may find that more than one household is present. A household is defined as one person or a group of people living in a dwelling unit, who (a) share cooking facilities and (b) share a living room, sitting room or a dining area.
A single address may also contain more than one dwelling unit, for example a house which has been split into 2 flats. A dwelling unit is a living space with its own front door, which can be either a street door or a door within a house or block of flats. Moreover, a single dwelling unit may include just one household or multiple resident households, for example 2 families living as 2 separate households in one house.
In England, addresses containing multiple dwelling units are not identified in the PAF and will not be detected until the interviewer has visited the address. For example, most apartments, whether in a block of flats or within a house, will be listed in their own right in the PAF. That is, these apartments are listed with their own address in the PAF, and assuming they meet the criteria of a single address (as defined above) they would be considered as one dwelling unit only. However, for some apartment blocks or houses that contain multiple dwelling units, the PAF will not list the individual addresses for each dwelling unit. Where this is the case, the interviewer will need to establish the different dwelling units that are part of the address that was given to them in their sample point. Furthermore, the PAF does not provide information on the number of households at a given address, and so the presence of other dwelling units is only detected when the interviewer visits the address.
Households residing at PAF-sampled addresses with multiple dwelling units or households, or both, will have had a lower chance of selection than others. While there are relatively few such addresses (1%), they account for a larger proportion of households, and these households tend to be rather different to others (poorer, younger, and smaller), so consequent biases may not be entirely trivial.
Interviewers must select one household to approach to take part at each sampled address. Interviewers are instructed to first establish the number of dwelling units at each sampled address. If there is more than one dwelling unit at the address, interviewers list these dwelling units in the electronic Address Record Form system (eARF) on their laptops so that the computer can randomly sample one of them. They then establish the number of households residing within the dwelling unit (whether it is the only dwelling unit at the address or the selected dwelling unit at an address with multiple dwelling units). Similarly, if there is more than one household, interviewers list them out in the eARF so that the computer can randomly select one of them.
Corrective weighting is then used to remove any bias arising from the lower chance of selection among dwelling units or households residing at multi-household addresses.
3.9 Ineligible (deadwood) addresses
The following types of address were classified as ineligible in 2025:
houses not yet built or under construction
demolished or derelict buildings or buildings where the address has “disappeared” when 2 addresses were combined into one
vacant or empty housing unit: housing units known not to contain any resident household on the date of the first contact attempt
a non-residential address: an address occupied solely by a business, school, government office or other organisation with no resident persons
residential accommodation not used as the main residence of any of the residents. This is likely to apply to second homes, seasonal, vacation or temporary residences, and these were excluded to avoid double counting
a communal establishment or institution: that is, an address at which 4 or more unrelated people sleep; while they may or may not eat communally, the establishment must be run or managed by the owner or a person (or persons) employed for this purpose
an address is residential and occupied by a private household(s), but does not contain any household eligible for the survey; it is very rare for a residential household not to be eligible for the NTS interview, exceptions include ‘Household of foreign diplomat or foreign serviceman living on a base’, addresses which are not the ‘Main residence’ of any of the residents and addresses where there are no residents aged 16 or over
an address out of sample: that is, cases where interviewers were directed not to approach a particular address; this is very rare and usually only occurs where an address should not have been listed on the original sampling frame
For further information about outcome coding, see section 4.16.
3.10 PSU-level variables
In addition to the information provided by members of the sampled households, the NTS also collects information measured at the PSU-level. The value of a PSU-level variable applies to all households living within that PSU. The PSU-level is therefore the highest level at which the data may be analysed, coming just above the Household level in the analysis hierarchy.