The review found no eligible studies for this question.
It is very uncertain whether monetary incentives: addition of monetary incentives changes retention [ compared with addition of a monetary reward (very low certainty).
It is very uncertain whether monetary incentives: addition of monetary reward changes retention [ compared with usual follow‐up (very low certainty).
Non‐monetary incentives: addition of pen showed no clear difference in retention [ compared with usual follow‐up (low certainty).
It is very uncertain whether maintaining participant engagement: newsletter changes retention [ compared with usual follow‐up (very low certainty).
Maintaining participant engagement: post‐it note showed no clear difference in retention [ compared with usual follow‐up (low certainty).
It is very uncertain whether behavioural interventions: theory informed cover letter changes retention [ compared with usual cover letter (very low certainty).
It is very uncertain whether impact of recruitment: optimised information changes retention [ compared with standard information (very low certainty).
Questionnaire design: addition of diary to usual follow‐up probably leads to lower retention [ compared with usual follow‐up (moderate certainty).
It is very uncertain whether data collection location and method: telephone follow‐up changes retention [ compared with postal questionnaire (very low certainty).
The intervention showed no clear difference in retention [ (low certainty).
Reminders: electronic reminder showed no clear difference in retention [ compared with usual follow‐up (low certainty).
It is very uncertain whether prompts: Electronic prompt changes retention [ compared with no prompt (very low certainty).
It is very uncertain whether prompts: Telephone prompt changes retention [ compared with usual follow‐up (very low certainty).
Prompts: personalised prompt showed no clear difference in retention [ compared with usual follow‐up (low certainty).
Monetary incentives: addition of monetary incentives may leads to higher retention [ compared with usual follow‐up (low certainty).