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Telephone reminders instead of postal reminders (ID RET22)

Evidence Summary

What is it?

Instead of receiving a postal reminder, participants receive a telephone call to remind them to participate.

Does it work?

Telephone reminders compared to postal reminders, may result in a large decrease in retention.

How big is the effect?

A decrease of -19% (95% confidence interval = -33% to -5%).

How certain are we?

GRADE Low certainty.


We recommend that trialists use telephone reminders to replace postal reminders only in the context of an intervention evaluation.

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 telephone reminders instead of postal reminders. The chart below shows the impact of an absolute increase of -19% (95% CI = -33% to -5%). Retention is now 46%, which means our best estimate is that you would now be 34 responses short.

Cumulative Meta-Analysis*

*Random effects model done using Comprehensive Meta-Analysis v4 ( Differences >0% favour the intervention. The GRADE assessment is low because of the imprecision of a single study.

Resource Bundle

How to Cite

Citation: Ostrovska B. Evidence pack– Retention: Telephone reminders compared to postal (RET22), 2023,

More Information

  1. This summary is from the Cochrane review of strategies to improve retention in randomised trials (
  2. The ‘Does it work?’ statement is structured according to effect size and GRADE certainty as per GRADE Guidelines 26 ( The statement is for large 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
v1.0 - 06/07/2023
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