Summary of findings 4. Data collection location and method: return postage.

Return postage compared with control intervention for trial retention
Patient or population: trial participants being followed up for data collection Settings: any setting Intervention: various return postage strategies Comparison: control intervention
Outcomes	Illustrative comparative risks* (95% CI)		Relative effect (95% CI)	No of Participants (studies)	Certainty of the evidence (GRADE)
	Assumed risk	Corresponding risk
	Various return postage strategies (such as free post versus second class stamp; high priority mail stamp versus usual postage; and personal form)	Standard return postage
Retention [follow‐up]	Lowa		RR 1.06 (0.99 to 1.15)	1543 (3)	⊕⊕⊝⊝ low
	25 per 100	27 per 100 (25 to 29)
	Mediuma
	50 per 100	53 per 100 (50 to 58)
	Higha
	80 per 100	85 per 100 (79 to 92)
*The basis for the assumed risk (e.g. the median control group risk across studies) is provided in footnotes. The corresponding risk (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.