Table 4.

Plans to proactively promote participant retention (n = 266, 36.8%)^a

ORRCA Domains	Number (%)	Examples of quotes from RCT protocols
A. Data collection
A2. Data collection frequency and timing	1 (0.4%)	Participants will be monitored monthly for signs or symptoms of adverse effects. One month later, the participants will be reviewed in the clinic and a possible side effect check list will be conducted. To prevent attrition and to assess adherence to treatment, telephone interviews will be conducted again at 2 months from administration of the drug
A3. Data collection location and method	12 (4.5%)	The survey will be available online via qualtrics (https://www.qualtrics.com/) with a direct link sent to participants. For those who prefer a hard copy, it will be posted with a return envelope
		If participants in either group miss their scheduled visit, and it cannot be rescheduled within 4 weeks of their prior visit, the PC clinician may conduct the visit through telephone within seven days from the missed visit
A5. Data collection during routine care	4 (1.5%)	To minimise loss to follow-up, assessments are timed to coincide with routine clinical follow-up
		The primary outcome point will be collected at the final outpatient clinic appointment and, as such, it is anticipated that missing data for the primary outcome will be low
B. Participants
B1. Reminders (Including repeat contacting of participants via phone, post, email etc.)	39 (14.7%) – reminders	Reminders Participants who do not complete and return the study questionnaires in the specified time period will be contacted by the research team via telephone or email, as a reminder about the study
		Another potential limitation for this trial is the attrition rate. It is possible to have a high rate of participant dropout and subsequently a significant loss of data. Therefore, reminders via telephone contact, email, and SMS will be used
Prompts	17 (6.4%) – prompts	Prompts The researcher will send reminder calls 3 days in advance to promote retention
		To promote participant retention, we plan training sessions in consultation with the participants and inform participants timely about the entire training schedule and the assessments
Unclear	2 (0.8%)	Unclear Phone or email reminders for completion of follow-up questionnaires were performed with phone or email prompts based on the tailored design method proposed by Dillman et al
		To optimise follow-up, multiple attempts will be made to contact participants including contacting their referring doctor and at a minimum we will aim to record vital status for all participants
B2. Monetary incentives – direct cash provided to participants/gift vouchers, prizes that are monetary		Conditional monetary incentives All families receive remuneration for their time: US $100 for completing the baseline assessment and US $100 for completing each of the 3 follow-up assessments, for a total of $400. Families can also earn a US $50 bonus if they complete all 4 assessments
Conditional Incentives	27 (10.2%)	To maximise participation and follow-up rates, we offered patients $20 for completing each questionnaire
Unconditional incentives	1 (0.4%)	Unconditional monetary incentives We recognise the importance of participant retention and will offer a voucher of £10 at recruitment
B3. Non-monetary incentives –entry to raffles for prizes that are non-monetary, completion of trial certificates, offering the controls the intervention at the end of the trial		Conditional non-monetary incentives The success of the intervention is strongly dependent on enjoyment and active participation. In order to motivate children’s active participation, the staff will use several strategies to celebrate success achieving the proposed objectives in both the healthy lifestyle education and the exercise programmes: celebrate and recognise their efforts, reward with smiley emoticons, etc. Children who complete the programme successfully will be rewarded with a certificate of completion
Conditional incentives	3 (1.1%)	Moreover, patients who complete the intervention will have an 8-h nutrition education programme for free
Unconditional Incentives	4 (1.5%)	Unconditional non-monetary incentives The primary purpose of employing an attention control intervention is to limit principals’ and teachers’ disappointment at not receiving the iPLAY intervention, thereby increasing participation during data collection at the post-intervention and maintenance phases To minimise loss to follow-up, all controls will be offered the intervention at study end
B4. Maintaining participant engagement	14 (5.3%)	Once a participant is included, every reasonable effort is made to prevent attrition through the entire study period. In addition to the planned visits, all participants have, during the last 2 years, received two letters in connection with milestones and holidays. Distribution of letters will continue throughout the entire study period
		Personal data will be used to contact the participant, to thank them for participating in the study, to facilitate the follow-ups at 6 months and 2 years of age, to co-ordinate the follow-ups and to disseminate the results of the study to participants
B7. Supporting participation	7 (2.6%)	Participants will be advised at the initial trial enrollment meeting to carefully consider the required investment of time and effort involved in the current project. This will be undertaken to minimise any negative impact associated with loss to follow-up, and consequently likely withdrawal can be made prior to randomisation
		The CRA will explain the study to these patients using comprehensive ethics committee-approved documents and patients will be given opportunity to ask questions and receive further information. This process is to ensure participants are fully informed of the possible burden of appointments and data collection on their time and to enhance retention and reduce loss to follow-up
B8. Contact information	3 (1.1%)	Minimising attrition in follow-up assessments is vital to ensure the success of this trial. The at-risk and transient nature of the target population makes this task more difficult. To mitigate this, participants will provide a collateral person who can be contacted. Participants will not be precluded from taking part in the study if they are not comfortable giving the details of collateral persons
		To increase follow-up data collection, clinicians will collect contact telephone numbers for the participant and at least two family members or caregivers
B12. Motivations and experience	1 (0.4%)	According to Danish research ethics legislation, we will inform the participants about their rights as voluntary subjects in a scientific trial and interview them about their motivation for participation. We do this to make participants consider participation thoroughly to diminish the likelihood of their dropping out
C. Sites and Site Staff
C4. Maintaining staff engagement	1 (0.4%)	News letters from the project were regularly sent to all PHCC managers and RCs at both intervention and control centres
E. Study Design
E1. Choice of study outcomes	1 (0.4%)	PCOMS could also reduce the number of dropouts and/or increased patient satisfaction, all leading to cost reduction
E4. Randomisation method	1 (0.4%)	Subjects are randomised on arrival in the operating theatre to minimise the risk of dropouts after randomisation
Combined strategies	128 (48.1%)
Protocols with multiple retention strategies (n = 128 (48.1%)) Top most common combined strategies to proactively promote retention
Strategy		Number of protocols (%)
	• Reminder • Data collection location and method	6 (4.7%) Example of quote Participants can choose to complete the questionnaires online or by using a paper questionnaire. Non-respondents will be contacted by telephone within 2 weeks. If they do not respond to this reminder, they will be sent a reminder letter within 2 weeks
	• Reminder • Monetary incentive	6 (4.7%) 4 of the these were conditional monetary incentives 2 were unclear if they were conditional or unconditional monetary incentives
		Example of quote Participants will receive up to three reminder emails if they do not complete the research questionnaires within the allocated time frame. If the questionnaires are still not completed participants will be offered $A20 to complete it. Participants who comply with all study procedures will receive $A50
	• Prompt • Monetary incentive	5 (3.9%) All conditional incentives
	• Monetary incentives • Data collection location and method • Reminder	5 (3.9%) 4 of these were conditional monetary incentives 1 was an unconditional monetary incentive
	• Monetary incentives • Supporting participation	4 (3.1%) 3 of these were conditional monetary incentives For 1 it was unclear whether it was a condition or unconditional monetary incentive
	• Supporting participation • Maintaining participant engagement	4 (3.1%)
	• Prompt • Reminder • Monetary incentive	4 (3.1%) All conditional monetary incentives
	• Prompt • Reminder	4 (3.1%)
	• Reminders • Maintaining participant engagement	3 (2.3%)
	• Non-monetary incentive • Monetary incentive	3 (2.3%) 2 were both conditional for both types of incentives 1 was a conditional monetary incentive combined with a non-monetary incentive but it was unclear if it was conditional or unconditional
	• Monetary incentives • Data collection location and method	3 (2.3%) All conditional monetary incentives
	• Prompt • Maintaining participant engagement • Monetary incentive	3 (2.3%) 2 were conditional monetary incentives 1 included both a conditional and unconditional monetary incentive

^aExcludes pilot and feasibility protocol data