Official Statistics

NTS mid-year estimates, year ending June 2023: Technical report

Published 25 April 2024

Applies to England

Chapter 1: Background

The National Travel Survey (NTS) provides up-to-date and regular information about personal travel within Great Britain and monitors trends in travel behaviour. First commissioned in 1965, since July 1988, the NTS has been a continuous survey (that is, fieldwork conducted on a monthly basis starting in January and ending in December).

Since January 2002, the Department for Transport (DfT) has commissioned the National Centre for Social Research (NatCen), an independent social research institute, as the contractor for the NTS. NatCen is responsible for questionnaire development, sample selection, data collection and editing, data file production and building the database. The DfT is responsible for data analysis, publication, and archiving.

Reporting of NTS results has been annual, based on a calendar year of data collection. However, starting from NTS 2023, an additional mid-year data delivery has been commissioned for 12 months’ worth of data collected from July of the previous survey year to June of the most recent survey year. As outlined in this technical note, this inaugural mid-year data delivery therefore combines the first 6 months of NTS 2023 with the last 6 months of NTS 2022, to give a more recent 12-month data delivery than the 2022 annual data. This mid-year data is currently being published as Official Statistics in Development.

This note briefly describes the differences as to sample design, survey content, and data preparation. The 2022 Technical Report contains full information on the overall methodology and approaches used on NTS. The detailed 2023 technical report will be provided with the annual data in the summer of 2024.

Chapter 2: Fieldwork

Historically, the NTS has used 2 data collection methods, face-to-face (F2F) interviewing using Computer Assisted Personal Interviewing (CAPI) and respondent self-completion of a 7-day travel record (issued as a paper diary). However, during July 2022 to June 2023, for respondents with COVID-19 or shielding or otherwise unable to take part face-to-face due to COVID-19, a phone back-up (PB) alternative was available. This phone back-up began in April 2022 and continued throughout 2023.

Due to ongoing challenges caused by the COVID-19 pandemic, a small proportion of the NTS 2022 points (also known as assignments) were issued using the Push-to-Telephone (P2T) approach, accounting for 12% of the sample for July to December 2022. This approach was used where it was not possible to allocate points to interviewers, and had also been in place at various stages prior to 2022 during the height of the pandemic (full details available in the 2022 Technical Report). The P2T approach involved the respondents actively opting-in to the study on the basis of a letter invitation only, and interviews and diary completions were always conducted via telephone. The remaining 88% of the July to December 2022 sample was issued as F2F with a phone back-up option (PB), as described above. The P2T approach was not used for any of the NTS 2023 survey year.

All approaches used the same CAPI questionnaire, but in cases where F2F was not possible the interviews were administered by telephone and the travel diaries were populated by interviewers collecting travel data from participants over the phone.

The overall sequence of tasks, from the initial sample selection to data analysis and reporting remained the same across face-to-face and telephone interviewing:

1. Sample selection from the Postcode Address File (PAF).

2. Questionnaire development and fieldwork preparation.

3. Fieldwork.

4. Data transmission and documents returned by interviewers.

5. In-house data input and editing.

6. Quality checking.

7. Data checking using SPSS.

8. Data file production, variable derivation and imputation and delivery to DfT.

9. Data analysis and reporting by DfT.

All aspects of the fieldwork sequence are as described in the 2022 Technical Report. This fieldwork sequence continued into NTS 2023.

Chapter 3: Sample selection and size

The combined 2022 and 2023 mid-year data is based on a random sample of 15,380 private households in England, drawn from the Postcode Address File (PAF).

The 2022 portion of sample represents 6,426 private households, and the 2023 portion of the sample represents 8,954 private households, for a combined total in this mid-year dataset of 15,380 private households. The 2023 sample is larger than the 2022 sample due to an increase in sample size implemented for the 2023 survey year. Full details of the sample increase will be outlined in the detailed 2023 Technical Report for the annual NTS data.

The 2022 and 2023 samples were drawn separately, but follow the same random sampling principles (including stratification and clustering), which are outlined in the 2022 Technical Report.

For 2023, to manage the increased sample size, the fieldwork points (also known as assignments) were increased from 17 addresses to 22. Additionally, more points were issued for the sample overall. Each of the 22 addresses in an interviewer’s point quota for 2023 continued to have a 7-day period (Travel Week) allocated to it, according to the usual rules of allocation, during which participants’ travel details were recorded. Interviewers were given a unique Travel Week Allocation Card with 22 Travel Weeks for each point, the start dates of which were spread evenly across a period of 31 days.

Chapter 4: Response

Tables 1.1 to 1.3 show the response rates for the combined 2022 and 2023 mid-year data, as well as the response rates for the 2 individual 6-monthly periods. The “set sample” figure represents the total number of addresses in the sample invited to take part in the study, and all figures shown are at the household level.

These tables demonstrate the response differences between the 2 periods, as well as the impact of the larger sample size for the 2023 portion of the combined mid-year sample (which resulted in an overall response rate that was closer to the 2023 response rate than the 2022 response rate). The response rate was 34% (2,804 households) for the first 6 months of 2023 compared to the lower response rate of 31% (1,840 households) for the last 6 months of 2022. Based on this, a response rate of 33% (4,644 households) was achieved for the combined mid-year period overall (July 2022 to June 2023).

Table 1.1: NTS response rates, mid-year data (July 2022 to June 2023)

Category	Achieved Sample	Achieved Sample Rate	Standard Response Rate
Set sample	15,380	100%	Not applicable
Ineligible or deadwood	1,212	8%	Not applicable
Unknown eligibility	1,927	13%	Not applicable
Eligible households	13,994	91%	100%
Fully co-operating	4,644	30%	33%
Partially co-operating	878	6%	6%
Refusal to co-operate and other unproductive	5,399	35%	49%
Non-contact	1,320	9%	12%

Table 1.2: NTS response rates, July to December 2022

Category	Achieved Sample	Achieved Sample Rate	Standard Response Rate
Set sample	6,426	100%	Not applicable
Ineligible or deadwood	469	7%	Not applicable
Unknown eligibility	1,127	18%	Not applicable
Eligible households	5,857	91%	100%
Fully co-operating	1,840	29%	31%
Partially co-operating	339	5%	6%
Refusal to co-operate and other unproductive	2,131	33%	50%
Non-contact	520	8%	12%

Table 1.3: NTS response rates, January to June 2023

Category	Achieved Sample	Achieved Sample Rate	Standard Response Rate
Set sample	8,954	100%	Not applicable
Ineligible or deadwood	743	8%	Not applicable
Unknown eligibility	800	9%	Not applicable
Eligible households	8,138	91%	100%
Fully co-operating	2,804	31%	34%
Partially co-operating	539	6%	7%
Refusal to co-operate and other unproductive	3,268	36%	47%
Non-contact	800	9%	12%

Note: The estimated number of eligible households is calculated by firstly deducting the ineligible or deadwood cases from the total sample, and then secondly deducting a proportion of the cases with unknown eligibility. The second deduction is made on the assumption that at least some of the addresses with unknown eligibility would have been ineligible or deadwood. The proportion used in this calculation is simply the inverse of the known eligibility rate.

Chapter 5: Data Processing amendments

As usual, the edited survey data was prepared for analysis and reporting before being delivered to DfT. However, as the data crosses 2 calendar years, changes had to be made to some protocols that are followed during post-processing.

Please refer to Chapter 5 of the 2022 technical report for full information on how the data was processed for NTS 2022.

Changes to these protocols for the combined 2022 and 2023 mid-year data processing are described below.

5.1 Creating household income semi-deciles and quintiles

The protocol was applied as per NTS 2022, with 2022 income mid-points applied to the NTS 2023 months. Income mid-points for 2023 were not available at the time of producing the data.

5.2 Adding Holidays data

The holidays database was extended in the usual manner to incorporate dates up to the end of September 2023, so that the NTS 2023 portion of the mid-year data can apply holiday status coding in the same way as the NTS 2022 portion of the data. This approach enables comparable analysis of trip data by travel day type for all months of the mid-year data delivery.

5.3 Adding concessionary travel data

The concessionary data does not change significantly year-on-year, so the 2022 concessions were applied to the NTS 2023 months for the mid-year data delivery.

Chapter 6: Weighting

The mid-year data combines the first 6 months from 2023 and the last 6 months from 2022. In practice, this includes a full year of survey data capturing travel behaviour and opinions across seasons. It was therefore possible to mostly adapt the weighting scheme previously used for NTS calendar year data and apply it to the combined mid-year data.

However, given that data was combined from 2 distinct surveys, a few adjustments were necessary. This was largely needed due to differing response rates across the 2 calendar years and hence an uneven proportion of responses between 2022 and 2023. Furthermore, a small proportion of the NTS 2022 points (12% of the sample for July to December 2022) were issued using the P2T approach, as described earlier in this technical note. The remaining 88% of the July to December 2022 sample was issued as F2F with a phone back-up option. NTS 2023, however, was carried out entirely F2F with a phone back-up option (PB).

The adjustments required for the mid-year data weighting (compared to the NTS 2022 weighting) are described in more detail below, but only affect the household participation non-response model and calibration stages of the interview weighting. No alterations were needed to the diary weighting method.

6.1 Household participation weights

The aim of the household participation weights is to attempt to reduce bias caused by systematic differences between the households that participated in the NTS (that is, for which a household interview was obtained) and those that did not. To generate the non-response weights, a logistic regression model was fitted with whether or not an eligible household participated as the outcome measure.

As 2 halves of independent NTS survey years were combined (which are known to not be biased in a consistent manner), the model was split by year whilst keeping the predictors consistent. This made it possible to account for year-specific non-response bias because of different survey modes and sample sizes. An alternative model split by quarter was fitted but subsequently disregarded due to weight efficiency reasons.

Due to the comparatively small proportion of P2T cases in the mid-year data overall, it was decided to not make any P2T specific adjustments to the weighting.

6.2 Calibration Weighting

For the combined 2022 and 2023 mid-year weights, the composite (household-level) weight from the previous stages was adjusted so that the distribution for groups defined by age and sex and region matched 2021 mid-year population estimates of household residents. The calibration adjustment was trimmed at the top and bottom 0.5%. This is in line with previous NTS years.

In addition to the above, each half from NTS 2022 and NTS 2023 was calibrated to 50% of the total to balance the 2 NTS surveys out as much as possible and account for seasonal differences. In the full NTS 2022 calendar year weighting, a similar approach was applied to even the responding sample out by quarter. As response rates by quarter within years were consistent in the mid-year data, a calibration to quarter was not necessary.

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