Table 2 Key findings, contamination risk and strategies for mitigating risk in future RCTs

 Staff movement, as some staff worked across intervention and control children centres.

Some staff trained in intervention delivery shared HENRY messages at control sites through other programmes and when providing advice to parents.

• Ensure research setting is transparent about staff movement at the beginning of the study, so this can be considered during randomisation.

• Ask staff not to share intervention content with control sites, and inform staff of the importance of reducing contamination so the importance of not sharing intervention content is understood

• Deliver intervention outside of standard practice and as part of a research project.

• Randomise at cluster level.

• Monitor staff movement at regular intervals

 Staff had knowledge of the programme prior to the study, and all staff was briefed on the study including the HENRY programme.

HENRY content was available to parents through children’s centre staff sharing advice and role modelling behaviours.

Some staff in control centres knew about the HENRY programme. Some staff made personal changes to be healthier, as a result of being involved in the study. This impacted on the information, advice and guidance given to parents, as this was based on personal experience.

• Provide training to centre staff on RCTs and the importance of minimising contamination.

• Record any prior knowledge of intervention amongst staff.

• keep control staff blinded as much as possible to intervention content

• Randomise at cluster level.

 Staff meetings involved staff from control and intervention centres discussing best practice and programmes being run.

Some staff were aware that they should filter what was shared at these meetings, however found it challenging to do so. Some staff discussed sharing about intervention content to help staff at other centres.

Staff routinely discussed programmes (including HENRY) at meetings.

• Encourage staff to not discuss the intervention at meetings, and to meet separately to discuss the programme with staff from the intervention arm only.

• Inform staff of the importance of minimising contamination, so the importance of not sharing intervention content is understood.

• Report any sharing of information within meetings to research team.

 Parents shared experiences of the HENRY programme with each other

The majority of parents did not attend/ have contact with parents from multiple centres and sharing was limited to parents who attend the same groups.

• Ask participants to not share intervention content and materials until after the study is completed.

• Asking parents to disclose what contact they have with other study centres or parents who attend other centres.

• Add ‘Contamination questions’ to test parental understanding of programme content and thus, identify contamination.

 Parents changed behaviour due to being recruited in study and aware of aim.

A few parents reported that they changed their behaviours once they had been recruited into the study as they knew that their weight was being monitored.

• Keep aim of study brief.

• Promise intervention to control group once study is completed.

• Ask control participants if they have changed their behaviours due to being recruited into the study.

 Parents shared existing content on social media.

Parents shared advice that other people/organisations had already posted on social media. Parents were unlikely to share about HENRY and usually shared articles or asked/ answered specific questions from other parents.

• Parents could be asked not to share about the intervention on social media for duration of study.

• Record information shared on social media.

 Some parents did not attend their local children’s centre and travelled further to attend one they preferred.

Parents did not have much contact with parents at other centres. If parents went to an intervention centre they may see HENRY displays/information.

• Do not encourage parents to attend other sites during study delivery.

• Monitor other centres that parents attend.

 Children's centers sharing parenting advice on social media

Children’s centres used social media to promote activities as part of routine practice. No parents reported knowledge of HENRY via social media or had shared about it.

• Recommendation that only intervention centres post about programme if social media is used.

• Recommend that if social media is to be used, social media posts promote the programme but do not share intervention content.

• Monitor if intervention facilitators share intervention information on social media.