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Providing participant information over telephone (ID REC17)

Evidence Summary

What is it?

Instead of a face-to-face meeting, participants are given information and consent is obtained over the telephone.

Does it work?

Providing information over the telephone may have little or no effect on recruitment.

How big is the effect?

A decrease of -7% (95% confidence interval = -18% to 5%).

How certain are we?

GRADE Low certainty.


We recommend that trialists provide participant information over the telephone in the context of an intervention evaluation.

How can I use this straight away?

See Resource bundle below for details on how to provide participant information over telephone.

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 providing participant information over the telephone. The chart below shows the impact of an absolute decrease of -7% (95% CI = -18% to 5%). Recruitment is now 23%, which means our best estimate is that 130 people would now need to be approached to recruit 30 of them.

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 and a wide crossing CI crossing RD=0.

Resource Bundle

How to Cite

Citation: Treweek S, Bruhn H, Gardner H. Evidence pack­– Recruitment: Providing participant information over telephone (REC17), 2023,

More Information

  1. This summary is from the Cochrane review of strategies to improve recruitment in randomised trials (
  2. It 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 ( The statement is for a moderate effect size and GRADE Low 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
v2.0 - 10/07/2023
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