Timing and value of participant vouchers to improve questionnaire return rates in the STADIA (STAndardised Diagnostic Assessment for children and adolescents with emotional difficulties) Trial

SWAT, incentives, vouchers, randomised controlled trials, young people, STADIA ALL Metrics - Views Downloads How to cite this article Holt G, Wyatt L, Partlett C et al. Timing and value of participant vouchers to improve questionnaire return rates in the STADIA (STAndardised Diagnostic Assessment for children and adolescents with emotional difficulties) Trial [version 1; peer review: 1 approved, 2 not approved]. NIHR Open Res 2025, 5:58 (https://doi.org/10.3310/nihropenres.13954.1) NOTE: If applicable, it is important to ensure the information in square brackets after the title is included in all citations of this article. Export Citation Sciwheel EndNote Ref. Manager Bibtex ProCite Sente Select a format first ▬ ✚ Research Article [version 1; peer review: 1 approved, 2 not approved] Grace Holt1, Laura Wyatt2, Christopher Partlett https://orcid.org/0000-0001-5139-3412 3, [...] Colleen Ewart https://orcid.org/0000-0001-5736-9800 4, Kirsty Sprange https://orcid.org/0000-0001-6443-7242 3, Alexandra Lang5, Kath Starr6, Alan Montgomery https://orcid.org/0000-0003-0450-1606 3, Kapil Sayal https://orcid.org/0000-0002-2050-4316 7,8Grace Holt1, Laura Wyatt2, [...] Christopher Partlett https://orcid.org/0000-0001-5139-3412 3, Colleen Ewart https://orcid.org/0000-0001-5736-9800 4, Kirsty Sprange https://orcid.org/0000-0001-6443-7242 3, Alexandra Lang5, Kath Starr6, Alan Montgomery https://orcid.org/0000-0003-0450-1606 3, Kapil Sayal https://orcid.org/0000-0002-2050-4316 7,8 PUBLISHED 10 Jul 2025 Author details Author details 1 Cancer Research UK Clinical Trials Unit, University of Birmingham, Birmingham, West Midlands, B15 2TT, UK 2 LifeArc Centre for Acceleration of Rare Disease Trials, University of Birmingham, Birmingham, England, UK 3 Nottingham Clinical Trials Unit, School of Medicine, University of Nottingham, Nottingham, Nottinghamshire, NG7 2RD, UK 4 STADIA Patient and Public Involvement co-lead, Institute of Mental Health, School of Medicine, University of Nottingham, Nottingham, England, NG7 2RD, UK 5 Human Factors Research Group, Faculty of Engineering, University of Nottingham, Nottingham, England, NG7 2RD, UK 6 Warwick Clinical Trials Unit, University of Warwick, Coventry, England, CV4 7AL, UK 7 Unit of Mental Health & Clinical Neurosciences, School of Medicine, University of Nottingham, Nottingham, England, UK 8 Institute of Mental Health, Nottinghamshire Healthcare NHS Foundation Trust, Nottingham, Nottinghamshire, UK 2 LifeArc Centre for Acceleration of Rare Disease Trials, University of Birmingham, Birmingham, England, UK 3 Nottingham Clinical Trials Unit, School of Medicine, University of Nottingham, Nottingham, Nottinghamshire, NG7 2RD, UK 4 STADIA Patient and Public Involvement co-lead, Institute of Mental Health, School of Medicine, University of Nottingham, Nottingham, England, NG7 2RD, UK 5 Human Factors Research Group, Faculty of Engineering, University of Nottingham, Nottingham, England, NG7 2RD, UK 6 Warwick Clinical Trials Unit, University of Warwick, Coventry, England, CV4 7AL, UK 7 Unit of Mental Health & Clinical Neurosciences, School of Medicine, University of Nottingham, Nottingham, England, UK 8 Institute of Mental Health, Nottinghamshire Healthcare NHS Foundation Trust, Nottingham, Nottinghamshire, UK Grace Holt Roles: Formal Analysis, Methodology, Writing – Original Draft Preparation, Writing – Review & Editing Roles: Formal Analysis, Methodology, Writing – Original Draft Preparation, Writing – Review & Editing Laura Wyatt Roles: Data Curation, Methodology, Project Administration, Writing – Review & Editing Roles: Data Curation, Methodology, Project Administration, Writing – Review & Editing Christopher Partlett Roles: Conceptualization, Methodology, Supervision, Writing – Original Draft Preparation, Writing – Review & Editing Roles: Conceptualization, Methodology, Supervision, Writing – Original Draft Preparation, Writing – Review & Editing Colleen Ewart Roles: Conceptualization, Methodology, Writing – Review & Editing Roles: Conceptualization, Methodology, Writing – Review & Editing Kirsty Sprange Roles: Conceptualization, Methodology, Writing – Review & Editing Roles: Conceptualization, Methodology, Writing – Review & Editing Alexandra Lang Roles: Conceptualization, Methodology, Writing – Review & Editing Roles: Conceptualization, Methodology, Writing – Review & Editing Kath Starr Roles: Conceptualization, Methodology, Project Administration, Supervision, Writing – Review & Editing Roles: Conceptualization, Methodology, Project Administration, Supervision, Writing – Review & Editing Alan Montgomery Roles: Conceptualization, Methodology, Supervision, Writing – Review & Editing Roles: Conceptualization, Methodology, Supervision, Writing – Review & Editing Kapil Sayal Roles: Conceptualization, Funding Acquisition, Methodology, Writing – Review & Editing Roles: Conceptualization, Funding Acquisition, Methodology, Writing – Review & Editing OPEN PEER REVIEW REVIEWER STATUS The STADIA trial evaluated the effectiveness of a standardised diagnostic assessment tool for children and young people (CYP) referred to Child and Adolescent Mental Health Services. Six- and 12-month follow-up questionnaires were completed by either CYP, parents/carers or both depending on the CYP’s age. A voucher was given upon completion of the questionnaires. A Study Within A Trial (SWAT) was embedded to investigate the effect of voucher timing and value on questionnaire return. Participants were randomly allocated to either: (A) receive a £10 voucher at each time point, conditional on completion of the questionnaire or (B) a £20 voucher on completion of their 12-month questionnaire. Outcomes included return of at least one questionnaire, questionnaire return at both 6- and 12-month follow-up, number of questionnaires returned (0,1,2) and time from randomisation to questionnaire return. Parent/carers, 11–15-year-olds and 16–17-year-olds were analysed separately. For each, between group comparisons were presented for each outcome along with 95% confidence intervals (CI). 284 participants were recruited (142 per arm). There was evidence of increased questionnaire return at both time points when giving two £10 vouchers, for 11–15-year-olds (adjusted risk ratio (ARR): 1.72 (95% confidence interval (CI):1.15 to 2.56)) and parents/carers (ARR 1.18 (95% CI:1.01 to 1.38)) but not for 16–17-year-olds (ARR 1.01 (95% CI:0.58 to 1.75)). Similarly, an increased number of questionnaires were returned for 11–15-year-olds (common odds ratio (COR): 3.25: (95% CI:1.41 to 7.52)) and parents/carers (COR 1.95 (95% CI:1.09 to 3.47)) but not for 16–17-year-olds (COR 1.18 (95% CI:0.29 to 4.77)). Smaller but regular incentives may increase questionnaire return within a trial rather than larger incentives at the end of follow-up. However, due to the small sample size, further research is required to confirm these findings both in the study population and wider population. The STADIA study tested the effectiveness of a standardised online information-gathering package in aiding the diagnosis of emotional disorders for children and young people (CYP) with emotional difficulties referred to the children and adolescent mental health services (CAMHS). Questionnaires were completed by CYP or their parents/carers, or both, depending on the CYP’s age, at six- and 12-months after joining the study. Participants received a voucher as a thank you for completing their questionnaires. We wanted to test whether the timing and value of the voucher made any difference in the number of questionnaires returned. Participants either received a £10 voucher upon returning their six- and 12-month questionnaire or one £20 voucher when they returned their 12-month questionnaire. We were interested in several factors 1) whether at least one questionnaire was returned (at 6- or 12-months), 2) whether both 6- and 12-month questionnaires were returned, 3) the number of questionnaires returned, 4) how long it took to return the questionnaire. The participants were split into the following groups: parent/carers, 11–15-year-olds and 16–17-year-olds. 142 families received a £10 voucher after completing both the 6- and 12-month questionnaires, and the other 142 families received one £20 voucher for completing the 12-month questionnaire. We found that offering a £10 voucher, at each time point, for completing the 6- and 12-month questionnaire increased the number of questionnaires returned for the 11–15-year-olds and parents/carers, compared to offering one £20 voucher at 12-months. The results suggest that giving participants more frequent, lower value vouchers may increase questionnaire return, compared to one higher value voucher at the end of the study. However, these results were based on a small number of young people referred to CAMHS, therefore further research is required to confirm whether these findings apply to the wider population. SWAT, incentives, vouchers, randomised controlled trials, young people, STADIA Corresponding Author(s) Christopher Partlett ([email protected]) Grant information: This project is funded by the National Institute for Health and Care Research (NIHR) under its [Health Technology Assessment Programme (HTA) (Grant Reference Number: NIHR-HTA-16/96/09 )]. The views expressed are those of the authors and not necessarily those of the NIHR or the Department of Health and Social Care. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Copyright: © 2025 Holt G et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. How to cite: Holt G, Wyatt L, Partlett C et al. Timing and value of participant vouchers to improve questionnaire return rates in the STADIA (STAndardised Diagnostic Assessment for children and adolescents with emotional difficulties) Trial [version 1; peer review: 1 approved, 2 not approved]. NIHR Open Res 2025, 5:58 (https://doi.org/10.3310/nihropenres.13954.1) First published: 10 Jul 2025, 5:58 (https://doi.org/10.3310/nihropenres.13954.1) Latest published: 10 Jul 2025, 5:58 (https://doi.org/10.3310/nihropenres.13954.1) The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Missing outcome data due to losses to follow-up reduces study power and may introduce bias affecting the validity of results. Strategies to improve the retention of trial participants throughout the follow-up period can reduce missing data, improving research efficiency and minimise the risk of bias (Brueton et al., 2013). A 2021 Cochrane review concluded that monetary incentives increase postal questionnaire response (Gillies et al., 2021). However, uncertainty remains around the most effective and efficient way to implement monetary incentives in trials (Edwards et al., 2009) (Khadjesari et al., 2011). For example, the Speed of Increasing milk Feeds Trial (SIFT) Study Within A Trial (SWAT), involving parents of preterm infants, concluded that providing unconditional vouchers prior to completion of a follow-up questionnaires improved response rate in comparison to giving vouchers after questionnaire completion (Juszczak et al., 2021). Whereas the Barrier Enhancement for Eczema Prevention (BEEP) SWAT found no evidence that either SMS text notification or including a voucher in a visit invitation letter had an effect on participant retention (Bradshaw et al., 2020). However, none of the previous studies have investigated timing of incentives. The STADIA trial evaluated the effectiveness of a standardised diagnostic assessment tool for children and young people (CYP) referred to Child and Adolescent Mental Health Services (Day et al., 2022). STADIA participants completed questionnaires at 6- and 12- months following randomisation. Originally, all STADIA participants were offered a single £20 voucher, conditional upon completion of the 12-month questionnaires. The 12-month time-point was chosen to reflect the end of participants’ involvement in the trial, with a single voucher payment selected for administrative simplicity and a higher value voucher considered to be preferable for participants. It was not known whether the offer instead of conditional voucher payments at both time-points may have an impact on the return of questionnaires. Patients and the public were directly involved in the development of the SWAT research question. Consultation with the STADIA Patient and Public Involvement (PPI) Lead (CE) and both the parent/carer panel and the STADIA Youth Lab (CYP panel) indicated that offering voucher payments at both time-points may promote engagement with the trial throughout the follow-up period. In response to this, the SWAT was devised to assess the effectiveness of voucher timing and value on questionnaire return rate. A two-arm, parallel randomised (1:1 allocation ratio) SWAT was embedded in the STADIA Trial (prospectively registered on ISRCTN, reference number: ISRCTN15748675). STADIA assessed the clinical and cost effectiveness of a Standardised Diagnostic Assessment tool called the Development And Well-Being Assessment (DAWBA) as an adjunct to usual clinical care in children and young people presenting with emotional difficulties referred to Child and Adolescent Mental Health Services (CAMHS). The primary outcome was a clinician-made diagnosis of an emotional disorder within 12 months from randomisation which was obtained from clinical records, however, secondary outcomes such as depression and anxiety symptoms in the child/young person and resource use information for the cost effectiveness analyses were obtained from 6- and 12-month participant completed questionnaires. The trial recruited 1,225 children/young people between 27th August 2019 and 17th October 2021 from eight NHS Trusts in England. Participant-reported outcome data were collected from participating parents/carers (P/C) and/or children/young people (depending in child age, see Table 1) at 6- and 12-months post-randomisation through online questionnaires, which participants were alerted to via email and / or text message. | Age of child/young person | <11 | 11-15 | 16-17 | |---|---|---|---| | Who is invited to complete research questionnaires? | Parent/carer only | Parent/carer Young person (optional) | Young person Parent/carer (optional) | The parent/carer was the primary participant for all children/young people aged <16. Children/young people aged 11–15 were also invited to participate alongside their parent/carer by completing their own questionnaires. Questionnaires for the child/young person aged 11–15 were sent via the parent/carer. Children/young people aged 16–17 years were the primary participant and were sent follow-up questionnaires directly for completion. For them, parent/carer participation was optional. If participating, they were considered the secondary participant and were sent their questionnaires directly. All host trial participants (both CYP and P/C) who were randomised between 07th May and 17th October 2021, were included in the SWAT. Follow-up questionnaires were returned between 07th November 2021 and 17th January 2023. Participants were allocated to receive either: a) one £10 voucher conditional on completion of the 6-month questionnaire plus another £10 voucher conditional on completion of the 12-month questionnaire or b) one £20 voucher conditional on completion of the 12-month questionnaire. Participants were informed of the conditional voucher and its value at each follow-up time-point when the questionnaires were sent. Reminders to complete both 6- and 12-month questionnaires also mentioned the voucher and its value for those who were allocated to receive two £10 vouchers, whereas, the 6-month reminder did not reference the 12-month voucher for those who were allocated to receive one £20 voucher at 12-months. e-voucher were used as they were cost effective, as well as easy to resource and distribute to participants via an email address. We did not specify a primary outcome for the SWAT as we did not anticipate the study would have sufficient power to detect small but important differences. However, we investigated the following outcomes: Return of at least one questionnaire during the follow-up period (yes/no) Questionnaire return at the 12-month follow-up time-point (yes/no) Questionnaire return at both follow-up time-points (yes/no) Number of follow-up questionnaires returned (0/1/2) The time from randomisation to return of the 6- and 12-month questionnaires. We estimated that during the lifetime of the SWAT, approximately 500 participants would be recruited to the host trial and therefore into the SWAT. Table 2 shows the smallest detectable risk difference for a binary outcome with 80% power for each group of respondents. This was based on a conservative baseline response rate of 50% and 5% two-sided significance with no adjustment for multiplicity. | Respondent | Approximate number randomised1 | Smallest detectable risk difference | |---|---|---| | Parent/carer | 450 (90%) | 13% | | Young people aged 11-15 | 124 (~25%) | 26% | | Young people aged 16-17 | 74 (~15%) | 33% | 1Participants will be either a child/young person AND parent/carer dyad, or a parent/carer only (for young people aged <16) or young person only (aged 16 or 17). The approximate numbers for each group of respondents assumed 500 participants would be randomised during the SWAT. The percentages denote the expected proportion of each type of respondent amongst all randomised participants. Following the host trial randomisation, participants had a secondary randomisation into the STADIA SWAT with a 1:1 allocation ratio using randomly permuted blocks (with block sizes of 2 and 4) stratified by host trial allocation. Child/young person and parent/carer dyads received the same allocation. The Trial Statistician created the allocation lists and then concealed randomisation was carried out automatically using the same online system as for the host trial. All participants were informed in the participant information sheet that if they completed the questionnaires, they would receive an e-voucher (value unspecified). Participants were not aware they were involved in an embedded SWAT. They were informed of the voucher denomination upon receipt of the questionnaire, which was at 6- and 12-months for the £10 group, and 12-months only for the £20 group. The STADIA trial management team were not blinded to the SWAT allocation. For each group of respondents, outcomes were analysed using appropriate mixed-effects regression models dependent on data type. For binary outcomes, mixed-effect logistic regressions were used. For number of questionnaires returned we used original regressions and then for time to event outcomes a cox proportional hazards model was used. The models adjusted for host trial allocation and included a random effect for recruiting site. Comparisons of binary outcomes were presented as both an absolute (risk difference) and relative (risk ratio) effect, along with 95% confidence intervals. Time to event outcomes were presented as hazard ratios and ordinal outcomes were presented as common odds ratios, with 95% confidence intervals presented for both. Analysis was carried out using Stata v18.0. The SWAT recruitment ran between 07 May and 17 Oct 2021, with a total of 284 participants recruited (142 in each arm) as shown in Figure 1. Recruitment ended when the sample size for the host trial was achieved. Total recruitment was lower than the 500 expected, however all were included in the final analysis. Baseline characteristics for each age group are displayed in Table 3. There were some imbalances across arms due to the small numbers recruited to the SWAT. There was evidence of increased questionnaire return at both 6- and 12-months when giving two £10 vouchers in comparison to one £20 voucher, for 11–15-year-olds and parent/carers but not for 16–17-year-olds (Table 4). Similarly, an increased number of questionnaires were returned by those given two £10 vouchers for 11–15-year-olds and parents/carers but not for 16–17-year-olds. No evidence of a difference was found between the two groups for any other outcomes. | Outcome | 11-15 years | 16-17 years | Parent/carer | ||| |---|---|---|---|---|---|---| | Two £10 vouchers (n = 50) | Single £20 voucher (n = 43) | Two £10 vouchers (n = 21) | Single £20 voucher (n = 14) | Two £10 vouchers (n = 124) | Single £20 voucher (n = 132) | | | Binary1 | |||||| | Return of at least one questionnaire during the follow-up period | |||||| | Yes | 44 [88%] | 32 [74%] | 18 [86%] | 10 [71%] | 115 [93%] | 113 [86%] | | Adjusted risk difference between groups (95% CI) | 12.65 (-3.09, 28.38) | 11.08 (-16.51, 38.67) | 7.51 (-0.66, 15.67) | ||| | Adjusted risk ratio between groups (95% CI) | 1.17 (0.96, 1.43) | 1.15 (0.80, 1.65) | 1.09 (0.99, 1.20) | ||| | Number of observations | 93 | 35 | 256 | ||| | Questionnaire return at 12-month follow-up time-point | |||||| | Yes | 39 [78%] | 28 [65%] | 14 [67%] | 10 [71%] | 105 [85%] | 103 [78%] | | Adjusted risk difference between groups (95% CI) | 12.20 (-6.13, 30.53) | -8.30 (-39.00, 22.39) | 6.88 (-3.03, 16.78) | ||| | Adjusted risk ratio between groups (95% CI) | 1.19 (0.91, 1.54) | 0.89 (0.57, 1.38) | 1.09 (0.96, 1.23) | ||| | Number of observations | 93 | 35 | 256 | ||| | Questionnaire return at both follow-up time points | |||||| | Yes | 36 [72%] | 18 [42%] | 13 [62%] | 8 [57%] | 99 [80%] | 90 [68%] | | Adjusted risk difference between groups (95% CI) | 29.93 (10.53, 49.33) | 0.36 (-32.65, 33.37) | 12.21 (1.15, 23.27) | ||| | Adjusted risk ratio between groups (95% CI) | 1.72 (1.15, 2.56) | 1.01 (0.58, 1.75) | 1.18 (1.01, 1.38) | ||| | Number of observations | 93 | 35 | 256 | ||| | Ordinal2 | |||||| | Number of follow-up questionnaires returned | |||||| | 2 | 36 [72%] | 18 [42%] | 13 [62%] | 8 [57%] | 99 [80%] | 90 [68%] | | 1 | 8 [16%] | 14 [33%] | 5 [24%] | 2 [14%] | 16 [13%] | 23 [17%] | | 0 | 6 [12%] | 11 [26%] | 3 [14%] | 4 [29%] | 9 [7%] | 19 [14%] | | Common odds ratio between groups (95% CI) | 3.25 (1.41, 7.52) | 1.18 (0.29, 4.77) | 1.95 (1.09, 3.47) | ||| | Number of observations | 93 | 35 | 256 | ||| | Time to event outcomes3 | |||||| | Time to 6-month questionnaire return (days) | |||||| | Mean [SD] | 210.2 [24.7] | 210.4 [23.0] | 208.3 [25.0] | 184.3 [2.1] | 204.2 [22.1] | 204.8 [20.8] | | Median [25th, 75th centile] | 201 [191, 221] | 209.5 [188, 229] | 202 [184, 224] | 184 [183.5, 185] | 199 [184, 215] | 202 [185.5, 218.5] | | Min, max | [181, 270] | [184, 254] | [181, 255] | [181, 188] | [181, 270] | [181, 270] | | n | 41 | 22 | 17 | 8 | 109 | 100 | | Adjusted hazard ratio between groups (95% CI) | 1.14 (0.67, 1.94) | 0.54 (0.19, 1.51) | 1.09 (0.83, 1.43) | ||| | Number of observations | 93 | 35 | 256 | ||| | Time to 12-month questionnaire return (days) | |||||| | Mean [SD] | 389.5 [24.8] | 387.5 [22.8] | 380.6 [26.5] | 386.7 [34.8] | 383.0 [21.6] | 385.1 [23.8] | | Median [25th, 75th centile] | 382 [371, 405] | 387 [367.5, 399] | 365 [365, 397] | 365 [365, 425] | 374 [367, 391] | 377 [365, 394] | | Min, max | [365, 457] | [365, 437] | [365, 446] | [365, 451] | [365, 457] | [365, 457] | | n | 39 | 28 | 14 | 10 | 105 | 103 | | Adjusted hazard ratio between groups (95% CI) | 1.00 (0.61, 1.64) | 1.37 (0.56, 3.32) | 1.12 (0.85, 1.47) | ||| | Number of observations | 93 | 35 | 256 | 1 Adjusted for trial allocation and random effect for recruiting site using a mixed effects logistic regression model. 2 Adjusted for trial allocation (fixed effect) and random effect for recruiting site using a multilevel ordered logistic regression model. 3 Adjusted for trial allocation using a Cox proportional hazards model. CI – Confidence Interval. SD – Standard Deviation. In this SWAT, we found evidence that giving two £10 vouchers on completion of both 6- and 12-month follow-up questionnaires increased return of both questionnaires, among 11–15-year-olds and parent/carer participants, in comparison to a one-off £20 voucher on completion of the 12-month questionnaire. There was no evidence of an effect among the 16–17-year-old group but this sub-sample was very small. We also found that 11–15-year-olds and parent/carers were more likely to return more questionnaires if they received two £10 vouchers rather than a single £20 voucher. Delays in approval and updating the trial database meant that the recruitment window for the SWAT was reduced with recruitment starting in May 2021, rather than February 2021 as originally planned. This meant that the sample size was smaller than expected therefore the SWAT was not powered to detect a difference for any of the outcomes, especially in the 16-17-year-olds group. The SWAT sample reflected the host trial population, which was representative of the target population of CYP seen by CAMHS in terms of key demographics (Sayal et al., 2025), therefore the conclusions made may not be generalisable out of this setting. Therefore, the results suggest that small and regular incentives may be more likely to increase retention within a trial rather than larger incentives at the end of follow-up, for both parent/carers and young people referred to CAMHS. Further research would be required to further strengthen these findings. Due to the results suggesting that providing £10 vouchers at each time point increases questionnaire return rates, trialists should consider a little and often approach with vouchers to encourage questionnaire return. This is also important for more complete capture of resource use and health economics data, which is crucial for trials evaluating cost effectiveness. However, again due to the small sample sizes and specific study population, further research is required to confirm these findings. Previous SWATs attempted to assess the effectiveness of providing financial incentives to improve retention within clinical trials (Cooke et al., 2013; Parker et al., 2020), however they have not assessed whether the timing and value of the incentives have an effect. This SWAT indicates that the timing of providing monetary incentives may play a role in improving return rates of participant questionnaires, but further SWATs in the area will help to enhance the evidence base. The STADIA (host) trial received ethical approval from the South Birmingham Research Ethics Committee (Ref. 19/WM/0133). The SWAT protocol was approved as per Substantial Amendment 04 (21 Mar 2021). Written informed consent to participate in the trial was received prior to randomisation. For young people aged 5–15 years, parents/carers with parental responsibility provided written informed consent, with an option for 11–15 year-olds to provide assent to participate. Young people aged 16–17 provided informed consent, and with their permission their parent/carer could also participate. Participants were informed in the Participant Information sheet that they would receive an e-voucher as a thank you for the extra time completing the questionnaires. If the child (aged 11–15 years) completed the questionnaire they would also be sent a voucher as a token of appreciation. The voucher denomination was not specified. The analysis datasets will be available to researchers upon request from the NCTU ([email protected]), a minimum of 12 months after publication of the main trial publication. Access to the data will be subject to review of a data sharing and use request by a committee including the chief investigator and sponsor and will only be granted upon receipt of a data sharing and use agreement. Any data shared will be de-identified which may impact on the reproducibility of published analyses. Nottingham Research Data Management Repository : STADIA SWAT Extended data, http://doi.org/10.17639/nott.7540 (Partlett, 2025). This project contains the following underlying data: CONSORT Flow Diagram STADIA SWAT STADIA SWAT text and e-mail contents Data is available under Creative Commons by Attribution license.

were reported as per the CONSORT guidelines for reporting the results of a Study Within A Trial (SWAT) (Arundel et al., 2024). We would like to acknowledge the contributions of both the STADIA PPI Parent/Carer Group and the STADIA Youth Lab Members in developing and refining the research question and Sebastian Moody for facilitating revisions to the manuscript. Faculty Opinions recommendedReferences - Arundel CE, Clark LK, Parker A, et al.: Trial forge guidance 4: a guideline for reporting the results of randomised Studies Within A Trial (SWATs). Trials. 2024; 25(1): 183. PubMed Abstract | Publisher Full Text | Free Full Text - Bradshaw LE, Montgomery AA, Williams HC, et al.: Two-by-two factorial randomised Study Within A Trial (SWAT) to evaluate strategies for follow-up in a randomised prevention trial. Trials. 2020; 21(1): 529. PubMed Abstract | Publisher Full Text | Free Full Text - Brueton VC, Tierney J, Stenning S, et al.: Strategies to improve retention in randomised trials. Cochrane Database Syst Rev. 2013; (12): MR000032. PubMed Abstract | Publisher Full Text | Free Full Text - Cook J, Watson A, McPherson G, et al.: SWAT 13: financial incentives to complete follow-up questionnaires in a randomised trial. Queens University Belfast, SWAT Repository Store. 2013. Reference Source - Day F, Wyatt L, Bhardwaj A, et al.: STAndardised DIagnostic Assessment for children and young people with emotional difficulties (STADIA): protocol for a multicentre randomised controlled trial. BMJ Open. 2022; 12(5): e053043. PubMed Abstract | Publisher Full Text | Free Full Text - Edwards PJ, Roberts I, Clarke MJ, et al.: Methods to increase response to postal and electronic questionnaires. Cochrane Database Syst Rev. 2009; 2009(3): MR000008. PubMed Abstract | Publisher Full Text | Free Full Text - Gillies K, Kearney A, Keenan C, et al.: Strategies to improve retention in randomised trials. Cochrane Database Syst Rev. 2021; 3(3): MR000032. PubMed Abstract | Publisher Full Text | Free Full Text - Juszczak E, Hewer O, Partlett C, et al.: Evaluation of the effectiveness of an incentive strategy on the questionnaire response rate in parents of premature babies: a randomised controlled Study Within A Trial (SWAT) nested within SIFT. Trials. 2021; 22(1): 554. PubMed Abstract | Publisher Full Text | Free Full Text - Khadjesari Z, Murray E, Kalaitzaki E, et al.: Impact and costs of incentives to reduce attrition in online trials: two randomized controlled trials. J Med Internet Res. 2011; 13(1): e26. PubMed Abstract | Publisher Full Text | Free Full Text - Parker A, Arundel C, Clark L, et al.: SWAT 180: the effectiveness and cost effectiveness of financial incentives for increasing participant retention rates in randomised trials. Queens University Belfast, SWAT Repository Store. 2020. Reference Source - Partlett C: STADIA SWAT extended data. Nottingham Research Data Management Repository. 2025. Publisher Full Text - Sayal K, Day F, Sprange K, et al.: SWAT 157: timing and value of participant vouchers to improve questionnaire return rates in the STADIA (STAndardised DIagnostic Assessment for children and adolescents with emotional difficulties) trial. Queens University Belfast, SWAT Repository Store. 2020. Reference Source - Sayal K, Wyatt L, Partlett C, et al.: The clinical and cost effectiveness of a STAndardised DIagnostic Assessment for children and adolescents with emotional difficulties: the STADIA multi-centre randomised controlled trial. J Child Psychol Psychiatry. 2025; 66(6): 805–820. PubMed Abstract | Publisher Full Text | Free Full Text Author details Author details 1 Cancer Research UK Clinical Trials Unit, University of Birmingham, Birmingham, West Midlands, B15 2TT, UK 2 LifeArc Centre for Acceleration of Rare Disease Trials, University of Birmingham, Birmingham, England, UK 3 Nottingham Clinical Trials Unit, School of Medicine, University of Nottingham, Nottingham, Nottinghamshire, NG7 2RD, UK 4 STADIA Patient and Public Involvement co-lead, Institute of Mental Health, School of Medicine, University of Nottingham, Nottingham, England, NG7 2RD, UK 5 Human Factors Research Group, Faculty of Engineering, University of Nottingham, Nottingham, England, NG7 2RD, UK 6 Warwick Clinical Trials Unit, University of Warwick, Coventry, England, CV4 7AL, UK 7 Unit of Mental Health & Clinical Neurosciences, School of Medicine, University of Nottingham, Nottingham, England, UK 8 Institute of Mental Health, Nottinghamshire Healthcare NHS Foundation Trust, Nottingham, Nottinghamshire, UK 2 LifeArc Centre for Acceleration of Rare Disease Trials, University of Birmingham, Birmingham, England, UK 3 Nottingham Clinical Trials Unit, School of Medicine, University of Nottingham, Nottingham, Nottinghamshire, NG7 2RD, UK 4 STADIA Patient and Public Involvement co-lead, Institute of Mental Health, School of Medicine, University of Nottingham, Nottingham, England, NG7 2RD, UK 5 Human Factors Research Group, Faculty of Engineering, University of Nottingham, Nottingham, England, NG7 2RD, UK 6 Warwick Clinical Trials Unit, University of Warwick, Coventry, England, CV4 7AL, UK 7 Unit of Mental Health & Clinical Neurosciences, School of Medicine, University of Nottingham, Nottingham, England, UK 8 Institute of Mental Health, Nottinghamshire Healthcare NHS Foundation Trust, Nottingham, Nottinghamshire, UK Grace Holt Roles: Formal Analysis, Methodology, Writing – Original Draft Preparation, Writing – Review & Editing Roles: Formal Analysis, Methodology, Writing – Original Draft Preparation, Writing – Review & Editing Laura Wyatt Roles: Data Curation, Methodology, Project Administration, Writing – Review & Editing Roles: Data Curation, Methodology, Project Administration, Writing – Review & Editing Christopher Partlett Roles: Conceptualization, Methodology, Supervision, Writing – Original Draft Preparation, Writing – Review & Editing Roles: Conceptualization, Methodology, Supervision, Writing – Original Draft Preparation, Writing – Review & Editing Colleen Ewart Roles: Conceptualization, Methodology, Writing – Review & Editing Roles: Conceptualization, Methodology, Writing – Review & Editing Kirsty Sprange Roles: Conceptualization, Methodology, Writing – Review & Editing Roles: Conceptualization, Methodology, Writing – Review & Editing Alexandra Lang Roles: Conceptualization, Methodology, Writing – Review & Editing Roles: Conceptualization, Methodology, Writing – Review & Editing Kath Starr Roles: Conceptualization, Methodology, Project Administration, Supervision, Writing – Review & Editing Roles: Conceptualization, Methodology, Project Administration, Supervision, Writing – Review & Editing Alan Montgomery Roles: Conceptualization, Methodology, Supervision, Writing – Review & Editing Roles: Conceptualization, Methodology, Supervision, Writing – Review & Editing Kapil Sayal Roles: Conceptualization, Funding Acquisition, Methodology, Writing – Review & Editing Roles: Conceptualization, Funding Acquisition, Methodology, Writing – Review & Editing Competing interests No competing interests were disclosed. Grant information This project is funded by the National Institute for Health and Care Research (NIHR) under its [Health Technology Assessment Programme (HTA) (Grant Reference Number: NIHR-HTA-16/96/09 )]. The views expressed are those of the authors and not necessarily those of the NIHR or the Department of Health and Social Care. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Copyright © 2025 Holt G et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. metrics VIEWS $counts.viewCount downloads Citations CITE how to cite this article Holt G, Wyatt L, Partlett C et al. Timing and value of participant vouchers to improve questionnaire return rates in the STADIA (STAndardised Diagnostic Assessment for children and adolescents with emotional difficulties) Trial [version 1; peer review: 1 approved, 2 not approved]. NIHR Open Res 2025, 5:58 (https://doi.org/10.3310/nihropenres.13954.1) NOTE: If applicable, it is important to ensure the information in square brackets after the title is included in all citations of this article. track receive updates on this article Track an article to receive email alerts on any updates to this article. Current Reviewer Status: ? Key to Reviewer Statuses VIEW HIDE ApprovedThe paper is scientifically sound in its current form and only minor, if any, improvements are suggested Approved with reservations A number of small changes, sometimes more significant revisions are required to address specific details and improve the papers academic merit. Not approvedFundamental flaws in the paper seriously undermine the findings and conclusions Version 1 VERSION 1 PUBLISHED 10 Jul 2025 Views 0 How to cite this report: Inghels M. Reviewer Report For: Timing and value of participant vouchers to improve questionnaire return rates in the STADIA (STAndardised Diagnostic Assessment for children and adolescents with emotional difficulties) Trial [version 1; peer review: 1 approved, 2 not approved]. NIHR Open Res 2025, 5:58 (https://doi.org/10.3310/nihropenres.15164.r36834) The direct URL for this report is: https://openresearch.nihr.ac.uk/articles/5-58/v1#referee-response-36834 https://openresearch.nihr.ac.uk/articles/5-58/v1#referee-response-36834 NOTE: it is important to ensure the information in square brackets after the title is included in this citation. Reviewer Report 11 Sep 2025 Maxime Inghels, University of Lincoln, Lincoln, England, UK Not Approved VIEWS 0 This study aims to evaluate the effect of the timing and value of a conditional incentive (£20 voucher) on questionnaire return rates at 6 and 12 months. Participants were randomised into two groups: (i) Group A, £10 for each completed ... Continue reading 2. Giles E, Robalino S, McColl E, Sniehotta F, et al.: The Effectiveness of Financial Incentives for Health Behaviour Change: Systematic Review and Meta-Analysis. PLoS ONE. 2014; 9 (3). Publisher Full Text 3. Haff N, Patel M, Lim R, Zhu J, et al.: The Role of Behavioral Economic Incentive Design and Demographic Characteristics in Financial Incentive-Based Approaches to Changing Health Behaviors: A Meta-Analysis. American Journal of Health Promotion. 2015; 29 (5): 314-323 Publisher Full Text 4. Inghels M, Kim H, Mathenjwa T, Shahmanesh M, et al.: Can a conditional financial incentive (CFI) reduce socio-demographic inequalities in home-based HIV testing uptake? A secondary analysis of the HITS clinical trial intervention in rural South Africa. Social Science & Medicine. 2022; 311. Publisher Full Text 5. Thornton R: The Demand for, and Impact of, Learning HIV Status. American Economic Review. 2008; 98 (5): 1829-1863 Publisher Full Text I confirm that I have read this submission and believe that I have an appropriate level of expertise to state that I do not consider it to be of an acceptable scientific standard, for reasons outlined above. Close This study aims to evaluate the effect of the timing and value of a conditional incentive (£20 voucher) on questionnaire return rates at 6 and 12 months. Participants were randomised into two groups: (i) Group A, £10 for each completed questionnaire (6 and 12 months), (ii) Group B, £20 for completing the 12-month questionnaire only. The authors conclude that smaller but regular incentives may increase questionnaire return rates compared with a larger incentive provided only at the end of follow-up. While the research question is relevant, I have several comments regarding the interpretation of the results, the analytical approach, and the theoretical framing. Interpretation of incentive timing vs. incentive value The conclusion that timing drives questionnaire return is not fully supported by the study design. In Group A, two questionnaires are incentivised (£10 each), whereas in Group B, only one questionnaire (12 months) is incentivised. Furthermore, participants in Group B were not informed at baseline that a £20 voucher would be offered for the 12-month questionnaire, nor were they reminded at the 6-month time point. As a result, the observed differences seem to reflect variations in incentive value at each time point rather than a pure effect of timing: - At 6 months: Group A (£10) vs Group B (£0). - At 12 months: Group A (£10) vs Group B (£20). Given this design, I would encourage the authors to reframe the findings as relating more to the value of incentives rather than timing. Stratification by age vs. decision-maker The authors chose to maintain original trial stratification by age groups. However, evidence suggests that age and demographics has little influence on the effect of financial incentives (see Haff et al., 2015; Inghels et al., 2022). For this analysis, stratification might have been more informative if based on the decision-maker (e.g., parent alone, child alone, or joint decision) or on the individual who benefits from the voucher. Such stratification could provide more meaningful insights into behavioural responses to incentives. Theoretical background and discussion The introduction and discussion would benefit from a stronger integration of financial incentive theories and behavioural economics literature. I would suggest addressing the following aspects: (i) Mechanistic rationale. Incentives often leverage present bias, the cognitive tendency to prefer immediate rewards over delayed ones (see Adams et al., 2014; Giles et al., 2014). Discussing this theoretical underpinning would provide stronger justification for the hypothesis and help with the interpretation of the findings. (ii) Similar 12-month return rates despite larger incentive. Despite a higher reward at 12 months (£20 vs £10), the return rates are similar between groups. This might be discussed in light of diminishing marginal returns on incentive value (see Thornton, 2008). (iii) Impact of non-disclosure of the 12-month Incentive. Because Group B participants were not informed about the 12-month incentive at baseline or at the 6-month point, they may have deprioritised or even disregarded the 12-month questionnaire, assuming no reward would be offered. In contrast, Group A participants, having received a voucher at 6 months, might reasonably expect another at 12 months. This potential information bias could partially explain the observed differences and should be acknowledged in the limitations section. Other study limitations and their potential implications for the results should be discussed in greater depth. (refer to 1,2,3,4,5,6) The authors conclude that smaller but regular incentives may increase questionnaire return rates compared with a larger incentive provided only at the end of follow-up. While the research question is relevant, I have several comments regarding the interpretation of the results, the analytical approach, and the theoretical framing. Interpretation of incentive timing vs. incentive value The conclusion that timing drives questionnaire return is not fully supported by the study design. In Group A, two questionnaires are incentivised (£10 each), whereas in Group B, only one questionnaire (12 months) is incentivised. Furthermore, participants in Group B were not informed at baseline that a £20 voucher would be offered for the 12-month questionnaire, nor were they reminded at the 6-month time point. As a result, the observed differences seem to reflect variations in incentive value at each time point rather than a pure effect of timing: - At 6 months: Group A (£10) vs Group B (£0). - At 12 months: Group A (£10) vs Group B (£20). Given this design, I would encourage the authors to reframe the findings as relating more to the value of incentives rather than timing. Stratification by age vs. decision-maker The authors chose to maintain original trial stratification by age groups. However, evidence suggests that age and demographics has little influence on the effect of financial incentives (see Haff et al., 2015; Inghels et al., 2022). For this analysis, stratification might have been more informative if based on the decision-maker (e.g., parent alone, child alone, or joint decision) or on the individual who benefits from the voucher. Such stratification could provide more meaningful insights into behavioural responses to incentives. Theoretical background and discussion The introduction and discussion would benefit from a stronger integration of financial incentive theories and behavioural economics literature. I would suggest addressing the following aspects: (i) Mechanistic rationale. Incentives often leverage present bias, the cognitive tendency to prefer immediate rewards over delayed ones (see Adams et al., 2014; Giles et al., 2014). Discussing this theoretical underpinning would provide stronger justification for the hypothesis and help with the interpretation of the findings. (ii) Similar 12-month return rates despite larger incentive. Despite a higher reward at 12 months (£20 vs £10), the return rates are similar between groups. This might be discussed in light of diminishing marginal returns on incentive value (see Thornton, 2008). (iii) Impact of non-disclosure of the 12-month Incentive. Because Group B participants were not informed about the 12-month incentive at baseline or at the 6-month point, they may have deprioritised or even disregarded the 12-month questionnaire, assuming no reward would be offered. In contrast, Group A participants, having received a voucher at 6 months, might reasonably expect another at 12 months. This potential information bias could partially explain the observed differences and should be acknowledged in the limitations section. Other study limitations and their potential implications for the results should be discussed in greater depth. (refer to 1,2,3,4,5,6) - Is the work clearly and accurately presented and does it cite the current literature? Partly - Is the study design appropriate and is the work technically sound? Partly - Are sufficient details of methods and analysis provided to allow replication by others? Partly - If applicable, is the statistical analysis and its interpretation appropriate? Partly - Are all the source data underlying the results available to ensure full reproducibility? Partly - Are the conclusions drawn adequately supported by the results? Partly

1. Adams J, Giles E, McColl E, Sniehotta F: Carrots, sticks and health behaviours: a framework for documenting the complexity of financial incentive interventions to change health behaviours. Health Psychology Review. 2014; 8 (3): 286-295 Publisher Full Text2. Giles E, Robalino S, McColl E, Sniehotta F, et al.: The Effectiveness of Financial Incentives for Health Behaviour Change: Systematic Review and Meta-Analysis. PLoS ONE. 2014; 9 (3). Publisher Full Text 3. Haff N, Patel M, Lim R, Zhu J, et al.: The Role of Behavioral Economic Incentive Design and Demographic Characteristics in Financial Incentive-Based Approaches to Changing Health Behaviors: A Meta-Analysis. American Journal of Health Promotion. 2015; 29 (5): 314-323 Publisher Full Text 4. Inghels M, Kim H, Mathenjwa T, Shahmanesh M, et al.: Can a conditional financial incentive (CFI) reduce socio-demographic inequalities in home-based HIV testing uptake? A secondary analysis of the HITS clinical trial intervention in rural South Africa. Social Science & Medicine. 2022; 311. Publisher Full Text 5. Thornton R: The Demand for, and Impact of, Learning HIV Status. American Economic Review. 2008; 98 (5): 1829-1863 Publisher Full Text Competing Interests: No competing interests were disclosed. Reviewer Expertise: I am a Health Demographer with expertise in study methodology and in applying financial incentives to change health behaviours. CITE HOW TO CITE THIS REPORT Inghels M. Reviewer Report For: Timing and value of participant vouchers to improve questionnaire return rates in the STADIA (STAndardised Diagnostic Assessment for children and adolescents with emotional difficulties) Trial [version 1; peer review: 1 approved, 2 not approved]. NIHR Open Res 2025, 5:58 (https://doi.org/10.3310/nihropenres.15164.r36834) The direct URL for this report is: https://openresearch.nihr.ac.uk/articles/5-58/v1#referee-response-36834 https://openresearch.nihr.ac.uk/articles/5-58/v1#referee-response-36834 NOTE: it is important to ensure the information in square brackets after the title is included in all citations of this article. Views 0 How to cite this report: Mitchell AJ. Reviewer Report For: Timing and value of participant vouchers to improve questionnaire return rates in the STADIA (STAndardised Diagnostic Assessment for children and adolescents with emotional difficulties) Trial [version 1; peer review: 1 approved, 2 not approved]. NIHR Open Res 2025, 5:58 (https://doi.org/10.3310/nihropenres.15164.r36400) The direct URL for this report is: https://openresearch.nihr.ac.uk/articles/5-58/v1#referee-response-36400 https://openresearch.nihr.ac.uk/articles/5-58/v1#referee-response-36400 NOTE: it is important to ensure the information in square brackets after the title is included in this citation. Reviewer Report 30 Aug 2025 Approved VIEWS 0 This is a very well-written report, I enjoyed reading it. I have a few minor comments that may improve the paper: I confirm that I have read this submission and believe that I have an appropriate level of expertise to confirm that it is of an acceptable scientific standard. Close - I've not heard of the term 'original regression' before. A Google search seems to show This is a very well-written report, I enjoyed reading it. I have a few minor comments that may improve the paper: - I've not heard of the term 'original regression' before. A Google search seems to show it's linear regression, perhaps use the term linear regression instead? - Please state the statistical method used to calculate the absolute risk difference. - A capital letter should be used for 'Cox' throughout (e.g. in the statistical analysis section Cox is not capitalised). - Is the work clearly and accurately presented and does it cite the current literature? Yes - Is the study design appropriate and is the work technically sound? Yes - Are sufficient details of methods and analysis provided to allow replication by others? Partly - If applicable, is the statistical analysis and its interpretation appropriate? Yes - Are all the source data underlying the results available to ensure full reproducibility? Yes - Are the conclusions drawn adequately supported by the results? Yes Competing Interests: No competing interests were disclosed. Reviewer Expertise: RCT methodology and statistics. CITE HOW TO CITE THIS REPORT Mitchell AJ. Reviewer Report For: Timing and value of participant vouchers to improve questionnaire return rates in the STADIA (STAndardised Diagnostic Assessment for children and adolescents with emotional difficulties) Trial [version 1; peer review: 1 approved, 2 not approved]. NIHR Open Res 2025, 5:58 (https://doi.org/10.3310/nihropenres.15164.r36400) The direct URL for this report is: https://openresearch.nihr.ac.uk/articles/5-58/v1#referee-response-36400 https://openresearch.nihr.ac.uk/articles/5-58/v1#referee-response-36400 NOTE: it is important to ensure the information in square brackets after the title is included in all citations of this article. Views 0 How to cite this report: Jacobson M. Reviewer Report For: Timing and value of participant vouchers to improve questionnaire return rates in the STADIA (STAndardised Diagnostic Assessment for children and adolescents with emotional difficulties) Trial [version 1; peer review: 1 approved, 2 not approved]. NIHR Open Res 2025, 5:58 (https://doi.org/10.3310/nihropenres.15164.r36664) The direct URL for this report is: https://openresearch.nihr.ac.uk/articles/5-58/v1#referee-response-36664 https://openresearch.nihr.ac.uk/articles/5-58/v1#referee-response-36664 NOTE: it is important to ensure the information in square brackets after the title is included in this citation. Reviewer Report 26 Aug 2025 Mireille Jacobson, University of Southern California, Los Angeles, California, USA Not Approved VIEWS 0 This paper addresses a very important topic – how to improve questionnaire return rates and, more specifically, whether financial incentives can be designed and deployed to improve return rates. The work assesses these issues by conducting a randomized study within ... Continue reading I confirm that I have read this submission and believe that I have an appropriate level of expertise to state that I do not consider it to be of an acceptable scientific standard, for reasons outlined above. Close This paper addresses a very important topic – how to improve questionnaire return rates and, more specifically, whether financial incentives can be designed and deployed to improve return rates. The work assesses these issues by conducting a randomized study within a trial (SWAT), which is in itself novel and important. The study design and analysis are a bit confusing and would benefit from more clarity and streamlining. I describe these issues and potential solutions below: - Incentive structure– the trial compares a £10 incentive offered for completion of a 6-month and, separately, a 12-month questionnaire to a £20 incentive offered for completion of the 12-month questionnaire. In other words, as I understand it, the £20 incentive can be received even if the participant does not complete the 6-month questionnaire. In this way, the experiment can be thought of as testing two different incentive structures: (1) no (£0) incentive versus a £10 incentive for completion of a 6-month questionnaire and (2) £10 vs. £20 incentive for completion of a 12-month questionnaire. An alternative approach might have been to test a £10 incentive offered for completion of each of the 6-month and 12-month questionnaires (as is currently done) compared to a £20 (or other amount) incentive for completion of both the 6-month and 12-month questionnaire. With a lump-sum £20 incentive for completion of both questionnaires, the set-up would vary and allow for estimating the impact of payment timing alone; changing the amount as well would vary and allow for estimating the impact of both timing and value. - Outcomes – Given the current set-up of the experiment, the main outcomes to consider are to my mind: (1) completion of a 6-month question, which compares the impact of no (£0) incentive to a £10 incentive and is not currently explicitly studied and (2) completion of the 12-month questionnaire, which compares incentives of differing amounts (a £10 vs. £20 incentive), which is in the paper. I would also consider an analysis of the impact of completing a 6-month questionnaire on the completion of the 12-month questionnaire, where instead of selecting on 6-month completion, you instrument for it with randomization to the £10 incentive scheme. This will only work if the incentive meaningfully increases completion of the 6-month questionnaire. Such an analysis might help put in perspective why the return at the 12-month follow-up appears higher (even if not statistically different from zero) under the £10 incentive scheme. - Clarity of payments – who was eligible for incentive payments is not entirely clear to me. Were optional reporters, i.e., 11–15 years old main study participants or parent/caregivers of 16–17 years olds, also eligible for incentive payments? If so, a family with a child in the 11–17 years old range was eligible for double the incentive amounts in their arm. Either way, this should be clarified, including how this was articulated to main study participants and their families. If a primary reporter did not complete a questionnaire but the optional person did, was the optional person paid? And, in that case, would the data from the optional person be used for the main study analysis? - Unit of analysis – If I understand correctly, the randomization was at the level of the child/dyad in the study, irrespective of who the reporter was. If so, I think the main analysis and the balance check should be performed at the level of the child/dyad, controlling for the primary person responsible for survey completion. In the balance check, you could compare the share of primary (non-optional) SWAT participants who were of different types, e.g., parent of a child under 11, parent of a child 11-15 years old, or 16-17 years old. In terms of the models, I would recommend estimating models that pool all primary reporters together and control for who the reporters was (as in the suggested balance check). This would greatly simplify the tables to review and help the reader in processing the outcomes. Instead of stratifying all the analyses, I would recommend doing secondary analyses that include interactions between the incentive scheme and reporter type. This last approach would allow you to test explicitly for differential impacts of the incentive scheme by reporter. - Is the work clearly and accurately presented and does it cite the current literature? Partly - Is the study design appropriate and is the work technically sound? Partly - Are sufficient details of methods and analysis provided to allow replication by others? Partly - If applicable, is the statistical analysis and its interpretation appropriate? Partly - Are all the source data underlying the results available to ensure full reproducibility? No - Are the conclusions drawn adequately supported by the results? Partly Competing Interests: No competing interests were disclosed. Reviewer Expertise: I am a health economist who has conducted numerous randomized studies of financial incentives. CITE HOW TO CITE THIS REPORT Jacobson M. Reviewer Report For: Timing and value of participant vouchers to improve questionnaire return rates in the STADIA (STAndardised Diagnostic Assessment for children and adolescents with emotional difficulties) Trial [version 1; peer review: 1 approved, 2 not approved]. NIHR Open Res 2025, 5:58 (https://doi.org/10.3310/nihropenres.15164.r36664) The direct URL for this report is: https://openresearch.nihr.ac.uk/articles/5-58/v1#referee-response-36664 https://openresearch.nihr.ac.uk/articles/5-58/v1#referee-response-36664 NOTE: it is important to ensure the information in square brackets after the title is included in all citations of this article. Alongside their report, reviewers assign a status to the article: - Approved - Approved with reservations - Not approved | Invited Reviewers | ||| |---|---|---|---| | 1 | 2 | 3 | | | Version 1 10 Jul 25 | read | read | read | Sign up for content alerts You are now signed up to receive this alert Alongside their report, reviewers assign a status to the article: Approved - the paper is scientifically sound in its current form and only minor, if any, improvements are suggested Approved with reservations - A number of small changes, sometimes more significant revisions are required to address specific details and improve the papers academic merit. Not approved - fundamental flaws in the paper seriously undermine the findings and conclusions Provide sufficient details of any financial or non-financial competing interests to enable users to assess whether your comments might lead a reasonable person to question your impartiality. 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