Evidence Summary

What is it?

Various strategies compared to usual practice for return postage, such as free post versus second class stamp; high priority mail stamp versus usual postage; and personal form.

Does it work?

Various return postage strategies may increase retention slightly.

How big is the effect?

An increase of 4% (95% confidence interval = 0% to 9%).

How certain are we?

GRADE Low certainty.


We recommend that trialists consider using strategies such as free post, high priority mail among others for return postage.

How can I use this straight away?

See Resource bundle below for details on various return postage strategies.

Practical Impact

Imagine initial retention is 65% of those approached. You have a trial with 100 participants that needs responses from 80 to meet its statistical power calculations.  Retention of 65% means that you will be 15 responses short (see chart below).


Now imagine using different strategy for return postage. The chart below shows the impact of an absolute increase of 4% (95% CI = 0% to 9%). Retention is now 69%, which means our best estimate is that you would now be only 11 responses short.

Cumulative Meta-Analysis*

*Random effects model done using Comprehensive Meta-Analysis v4 (www.meta-analysis.com).
Differences >0% favour the intervention. The GRADE assessment is low because of imprecision.

Resource Bundle


How to Cite

Citation: Ostrovska B. Evidence pack– Retention: Various return postage strategies (RET24), 2023, https://www.trialforge.org/retention-sector/various-return-postage-strategies-id-ret24/.

More Information

  1. This summary is from the Cochrane review of strategies to improve retention in randomised trials (https://www.cochranelibrary.com/cdsr/doi/10.1002/14651858.MR000032.pub3/full).
  2. The ‘Does it work?’ statement is structured according to effect size and GRADE certainty as per GRADE Guidelines 26 (https://doi.org/10.1016/j.jclinepi.2019.10.014). The statement is for small effect size and Low GRADE certainty.
  3. The recommendation statement is the consensus view of the authors of this summary based on the GRADE certainty and features of the trials contributing to the evidence.
  4. If you have any questions contact info@trialforge.org.
Version: v1.0 - 06/07/2023

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