Evidence Summary

What is it?

Questionnaires are sent by first class post as opposed to second class.

Does it work?

First class postage for outward mail may increase retention slightly.

How big is the effect?

An increase of 2% (95% confidence interval = -4% to 8%).

How certain are we?

GRADE Low certainty.


We recommend that trialists consider using first class postage for sending questionnaires.

How can I use this straight away?

See Resource bundle below for support materials.

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 first class postage for outward mail. The chart below shows the impact of an absolute increase of 2% (95% CI = -4% to 8%). Retention is now 67%, which means our best estimate is that you would now only be 13 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 unclear risk bias and the imprecision of a single study.

Resource Bundle


How to Cite

Citation: Ostrovska B., Treweek S. Evidence pack– Retention: First class postage (RET8), 2023, https://www.trialforge.org/retention-sector/first-class-postage-id-ret8/.

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 - 05/07/2023

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