Keywords
randomised controlled trial, embedded trial, postal questionnaire, response rate, pen
This article is included in the Studies Within A Trial (SWAT) collection.
randomised controlled trial, embedded trial, postal questionnaire, response rate, pen
Randomised controlled trials (RCTs) are key to evaluating the effectiveness of interventions and often use postal questionnaires to collect outcome data. However, low response rates can limit the validity of the trial findings by reducing the power of the study and introducing bias1.
Numerous strategies to increase response rates have been studied2,3 including sending a pen with the questionnaire. The pen acts both as a facilitator to aid completion of the questionnaire, and an incentive to return it4,5. The effectiveness of this intervention is equivocal with some studies reporting an increase in response rate5–7 whilst others failed to show a positive impact4,8. These studies displayed considerable heterogeneity and only two were embedded in RCTs6,7. A Study within a Trial (SWAT) is a self-contained study embedded within a host trial that can be used to evaluate strategies designed to improve trial efficiency9. This SWAT evaluated the effectiveness of enclosing a pen with a follow-up postal questionnaire on response rates in the SSHeW trial10.
This two-armed RCT was embedded in the SSHeW trial, a trial evaluating the effectiveness of slip-resistant footwear to reduce slips in NHS staff10. The SSHeW trial was registered (ISRCTN 33051393) and the trial protocol has been published10.
The SWAT was conducted in seven NHS Trusts in England and included all eligible participants in the SSHeW trial who were due to be sent their 14-week postal questionnaire between 04.07.2018 and 12.02.2019.
The intervention group were sent a York Trials Unit, University of York branded pen with their questionnaire. The control group did not receive a pen.
The SWAT outcomes are outlined in Table 1.
As is usual with an embedded trial, a formal sample size calculation was not undertaken as the sample size was determined by the number of participants due to receive their 14-week questionnaire.
We anticipated that randomising 2,000 participants into the SWAT would provide 80% power to detect an absolute difference of 6% (two-sided α=0.05) in response rates between the two groups, assuming a control rate of 60%.
Participants were allocated to either the intervention (pen) or control (no pen) group using simple randomisation in a 1:1 ratio. The allocation sequence was generated by the SSHeW trial statistician, who was not involved in sending out the questionnaires.
Participants were not aware of their involvement in this SWAT but due to the nature of the intervention participants and study team members could not be blinded to group allocation.
This SWAT was approved by the Department of Health Sciences Research Ethics Committee at the University of York and the Health Research Authority (HSRGC/2016/187/A).
Data were analysed using Stata version 1511 on an intention-to-treat basis, using two-sided tests at the 5% significance level. The models used for each outcome are given in Table 2, the values associated with the pen allocation from each model is presented with its 95% confidence interval and p-value. All models were adjusted for main trial group allocation (slip-resistant footwear or wait-list control) and pen sub-study allocation (pen or no pen).
The total cost of a standard SSHeW questionnaire pack was £2.42 (envelope and postage: £0.86; questionnaire and cover letter: £0.65; pre-paid envelope and postage: £0.91). The additional cost of including a pen was £0.32. The cost analysis incorporates the changes in number of questionnaires returned and reminders required.
A total of 1466 participants were included in the SWAT (pen, n=733; no pen, n=733). In total, 13 participants withdrew from the main SSHeW trial after they had been randomised into the SWAT but before being sent their follow-up questionnaire, leaving 1453 participants (Figure 1). Baseline characteristics are summarised descriptively in Table 3.
Results are presented in Table 4. Overall, 962 (66.2%) questionnaires were returned (pen, 67.7%; no pen, 64.7%) and an average of 4.9/5 items were completed. There was no evidence of a difference in return rate between the groups (OR 1.15, 95% CI 0.92 to 1.43, p=0.22), nor number of items completed (AMD -0.01, 95% CI 0.06 to 0.05, p=0.77).
OD, odds ratio; HR, hazards ratio; AMD, adjusted mean difference
There was weak evidence of a difference, in favour of the pen group, in both time to return (median time to return 15 vs 18 days; HR 1.12, 95% CI 0.98 to 1.27, p=0.09) (Figure 2), and in the proportion of participants requiring a reminder (OR 0.83, 95% CI 0.68 to 1.02, p=0.08).
A 3% difference in questionnaire response rate and an absolute difference in the percentage of participants who required a reminder of 1.1% were found. Considering these to be true effects, in order to receive one additional questionnaire, 33 participants would have to be sent a pen, at a cost of approximately 33x32p=£10.56. Approximately 91 participants would need to be sent a pen to prevent one reminder mailing and therefore to save £2.42. Hence, roughly one reminder is required per three retained participants, and the cost per retained participant is approximately £10.
Whilst the results of all outcomes in this SWAT favoured the pen group, we found that the addition of a pen did not statistically significantly increase the response rate to, or completeness of, a follow-up questionnaire sent at 14 weeks post-randomisation among participants of the SSHeW trial. There was some evidence of a reduction in time to response and the number of reminders required.
It may be that, in this group of participants, the pen failed to act as a facilitator or was not a sufficient incentive to return the questionnaire, given the fact that participants in the trial already received a free pair of shoes (although offer of shoes was not conditional on returning the questionnaire).
However, the trial ultimately only had about 40% power to detect a difference of 3% in response rates (from 64.7 to 67.7%) and is therefore at risk of a type II error. Another potential weakness is that, due to the select population of healthcare workers, the results may not be generalisable to other populations or contexts.
The strength of this study is that it was a randomised trial.
This SWAT suggests that enclosing a pen in a questionnaire mail out may be an effective method to increase response rates but was likely underpowered to detect a statistically significant difference of the 3% observed. Since pens are inexpensive, even a small difference is likely to be cost-effective. The results contribute to the body of evidence regarding this intervention and may be included in future meta-analyses to improve power.
Open Science Framework: SSHeW Trial Pen SWAT, https://doi.org/10.17605/OSF.IO/YQ76U12.
Open Science Framework: CONSORT checklist for ‘Enclosing a pen to improve response rate to postal questionnaire: an embedded randomised controlled trial’, https://doi.org/10.17605/OSF.IO/YQ76U12.
Data are available under the terms of the Creative Commons Zero "No rights reserved" data waiver (CC0 1.0 Public domain dedication).
The authors would like to thank the embedded trial participants who returned their trial documentation and the data administration staff at the YTU who worked to support the study.
The paper was written by the authors on behalf of the SSHeW Trial team, which is made up of the following individuals: Emily Bain (Health and Safety Executive); Sarah Cockayne (University of York); Rachel Cunningham-Burley (University of York); Caroline Fairhurst (University of York); Gillian Frost (Health and Safety Executive); Catherine Hewitt (University of York); Heather Iles-Smith (Leeds Teaching Hospital NHS Trust); Mark Liddle (Health and Safety Executive), Misbah Mogradia (Health and Safety Executive); David Torgerson (University of York), Michael Zand (Health and Safety Executive) and others. With the exception of Emily Bain and Misbah Morgradia, who supported Health Economic Evaluation for the host SSHeW Trial, all members of the SSHeW study team were involved in the designing this SWAT.
We would also like to acknowledge the support of individuals from the PROMETHEUS programme.
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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?
Yes
If applicable, is the statistical analysis and its interpretation appropriate?
Partly
Are all the source data underlying the results available to ensure full reproducibility?
Yes
Are the conclusions drawn adequately supported by the results?
Partly
References
1. Treweek S, Bevan S, Bower P, Briel M, et al.: Trial Forge Guidance 2: how to decide if a further Study Within A Trial (SWAT) is needed.Trials. 2020; 21 (1): 33 PubMed Abstract | Publisher Full TextCompeting Interests: No competing interests were disclosed.
Reviewer Expertise: Trial operations and recruitment
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?
Yes
If applicable, is the statistical analysis and its interpretation appropriate?
I cannot comment. A qualified statistician is required.
Are all the source data underlying the results available to ensure full reproducibility?
Yes
Are the conclusions drawn adequately supported by the results?
Partly
Competing Interests: Hanne Bruhn: None. Anne Duncan: None. Magaly Aceves Martins: I am participating in the Cochrane Review Update on Strategies to improve retention in randomised trials.
Reviewer Expertise: Hanne Bruhn: Health Services Research and Trial Methodology. Anne Duncan: Trial Management and conduct and Trial Methodology. Magaly Aceves Martins: Nutrition and systematic reviewing.
Alongside their report, reviewers assign a status to the article:
Invited Reviewers | ||
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Version 1 09 Jun 20 |
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