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).
Risk‐based showed no clear difference in combined outcome of proportion of participants with major or… compared with extensive on‐site monitoring (moderate certainty).
The review found no eligible studies for this question.
It is very uncertain whether central and local monitoring only changes impact of the monitoring strategy on compared with central and local monitoring with on‐site visits (very low certainty).
The review found no eligible studies for this question.