Evidence Summary

What is it?

Telephoning people who do not respond to mailed invitations to take part in a trial.

Does it work?

Telephone reminders probably increase recruitment.

How big is the effect?

An increase of 6% (95% confidence interval = 3% to 9%).

How certain are we?

GRADE High certainty.


We recommend that trialists consider using telephone reminders, ideally as part of a SWAT evaluation if the trial has higher initial recruitment (i.e. over 10%).

How can I use this straight away?

See Resource bundle below for details of how to use telephone reminders or build SWAT evaluation of them into your trial.

Practical Impact

Imagine a trial that needs to recruit 30 participants and initial recruitment is 30% of those approached. This means you’d need to approach 100 people to recruit 30 of them (see chart).


Now imagine using telephone reminders. The chart below shows the impact of an absolute increase of 6% (95% CI = 3% to 9%). Recruitment is now 36%, which means our best estimate is that 83 people would now need to be approached to recruit 30 of them.

Cumulative Meta-Analysis*

*Random effects model done using Comprehensive Meta-Analysis v3 (www.meta-analysis.com).
Differences >0% favour the intervention. GRADE assessment is high. This result applies to trials that have low underlying recruitment. We are less certain for trials that start out with moderately good recruitment (i.e. over 10%).

Resource Bundle


How to Cite

Citation: Treweek S, Bruhn H, Gardner H. Evidence pack– Recruitment: Telephone reminders (Rec4), 2021, https://www.trialforge.org/recruitment-sector/telephone-reminders-to-non-respondents/.


More Information

  1. This summary is from the Cochrane review of strategies to improve recruitment in randomised trials (https://doi.org/10.1002/14651858.MR000013.pub6).
  2. The summary was prepared with financial support from Evidence Synthesis Ireland.
  3. 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 moderate effect size and High GRADE certainty.
  4. 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.
  5. If you have any questions contact info@trialforge.org.
Version: v1.0 - 21/12/2021

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