Summary of findings 15. Behavioural interventions: theory informed cover letter vs usual cover letter.

Theory informed cover letter compared with usual cover letter for trial retention
Patient or population: trial participants being followed up for data collection Settings: any setting Intervention: theory informed cover letter Comparison: usual cover letter
Outcomes	Illustrative comparative risks* (95% CI)		Relative effect (95% CI)	No of Participants (studies)	Certainty of the evidence (GRADE)
	Assumed risk	Corresponding risk
	Theory informed cover letter	Usual cover letter
Retention [follow‐up]	Lowa		RR 1.05 (0.98 to 1.12)	3343 (4)	⊕⊝⊝⊝ very low
	25 per 100	26 per 100 (25 to 28)
	Mediuma
	50 per 100	53 per 100 (49 to 56)
	Higha
	80 per 100	84 per 100 (78 to 90)
*The basis for the assumed risk (e.g. the median control group risk across studies) is provided in footnotes. The effect of a theory informed cover letter (and its 95% confidence interval) is based on the assumed risk in the comparison group and the relative effect of the intervention (and its 95% CI). CI: confidence interval; RR: risk ratio.
GRADE Working Group grades of evidence High certainty: we are very confident that the true effect lies close to that of the estimate of the effect. Moderate certainty: we are moderately confident in the effect estimate; the true effect is likely to be close to the estimate of the effect, but there is a possibility that it is substantially different. Low certainty: our confidence in the effect estimate is limited; the true effect may be substantially different from the estimate of the effect. Very low certainty: we have very little confidence in the effect estimate; the true effect is likely to be substantially different from the estimate of effect.
a We selected low, medium and high illustrative retention levels of 25%, 50% and 80% based on prior experience with trial retention and evidence from the literature. For example, it has been previously stated that it is common for up to 20% trial participants to drop out before the trial finishes (Walsh 2015), as such we set the upper limit of good retention as 80%. The other extreme of 25% was informed by evidence that some trials (largely internet based) can have retention as low as 10% to 25% (Murray 2009). The mid point of 50% was a judgement made by the review team and was deemed appropriate given the evidence for the other parameters.