Trial design: The general methodology of this SWAT within the GRASP trial was guided by methodology developed and published by START (Systematic Techniques for Assisting Recruitment to Trials) (20, 21). Participants were randomised (1:1) to receive one of two interventions: 1) a personalised text message which included their name (intervention group), or 2) a standard text message (control group). Ethical approval was obtained from the Berkshire B Research Ethics Committee (REC) in the form of a substantive amendment to the host trial (REC Ref: 16/SC/0508; Integrated Research Application System (IRAS) ID 199243).
Participants: All participants in the PROMPTS SWAT were consented and enrolled from the GRASP trial, which acted as the host trial. In addition to meeting the inclusion criteria for the GRASP trial, the following inclusion criteria were applied:
- Participants needed to have the use of a mobile telephone.
- Participants were willing to provide a mobile telephone number and consented for contact to be made by the GRASP trial team using this number.
Messages were sent via a secure third-party text message gateway software (University of Oxford SMS service), so participants were also asked for consent to share their data with this third-party software company to allow the messages to be sent to them. Participants who did not provide a mobile number or did not consent to receive texts were excluded from the PROMPTS SWAT.
Interventions: Participants were randomised to receive either a personalised text message, which included their name (intervention group), or a standard text message (control group). The text message was sent to trial participants at the same time as their 6-month follow-up postal questionnaire was sent by the trial team, therefore it would arrive a few days before the participant received their follow-up questionnaire. For anyone (non-responders) who did not return their questionnaire pack at 6-months within 3 weeks of posting out, there then was a cascade of actions that followed:
- a reminder follow-up questionnaire, was posted 3 weeks after the initial questionnaire was sent
- a phone call to the participant was made 5 weeks after the initial questionnaire was sent
- an email to the participant inviting them to complete the questionnaire online was sent 8 weeks after the initial questionnaire had been posted.
Each text message contained the same core information. Recipients were reminded about the arrival of the questionnaire, about the importance of their responses and to return the postal questionnaire as soon as possible. The wording of the text messages in the intervention and control and groups are outlined in Box 1. For participants in the intervention group, text messages were customised using their name, according to how they preferred to be addressed e.g. Mr Smith, John Smith or John.
Outcomes: The primary outcome measure was questionnaire response rate, defined as the proportion of GRASP follow-up questionnaires returned by participants. The secondary outcome measures included: time to response (defined as the number of days which elapsed between the GRASP follow-up questionnaire being mailed out to participants and the questionnaire recorded as being returned to the GRASP trial team); the proportion of participants sent a reminder follow-up questionnaire; the cost of the text message intervention.
Randomisation and blinding: Participants were assigned a unique trial identification (ID) number by the GRASP trial team. A computer generated randomisation list was used to list all participants who provided a mobile telephone number. OCTRU’s Registration / Randomisation and Management of Product (RRAMP) web-based system was used to randomise and assign the allocation; the trial administrative team performed all randomisations once eligibility was confirmed. Participants were randomly allocated (1:1) to the intervention and control group. Generation of the allocation sequence and assignment of the intervention and control groups was undertaken independently by a researcher not involved with the delivery of the text messages. To avoid imbalance, block randomisation with equal probabilities of assignment to the intervention and control groups was be used. GRASP trial participants were blinded to the nature and objectives of the PROMPTS SWAT.
Sample size: As is usual with an embedded trial within a trial, no formal power calculation was undertaken as the sample size was constrained by the number of participants included in the GRASP trial receiving follow-up questionnaires and consenting to use of their mobile telephone number.
Analysis: All eligible participants were included in the analysis in accordance with the intention-to-treat design. The analyses were conducted in Stata 15.1 (StataCorp). Questionnaire response rates, and whether a reminder follow-up questionnaire was sent were compared between using a chi-square test and reported as risk ratios and 95% confidence intervals (CI). The time to return of the questionnaire was plotted using a Kaplan-Meier survival curve and estimated Cox proportional hazard ratios (HR) were calculated for the overall response and those returning the initial questionnaire and the log-rank test used to compare the Personalised and Standard text groups. The cost of the text message intervention was also calculated. Research staff costs were not calculated as the follow-up of participants was undertaken during the normal time on the host trial. We performed two post-hoc subgroup analyses for age (under 65 years / 65 years and older) and gender (male / female).