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Questionnaire covering issues relevant to the trial (ID REC21)

Evidence Summary

What is it?

questionnaire that covers issues relevant to the trial (e.g., a home safety questionnaire for an injury prevention trial) is added to the trial invitation pack.

Does it work?

Enclosing a questionnaire covering issues relevant to trial with the invitation probably increases recruitment.

How big is the effect?

An increase of 18% (95% confidence interval = 16% to 20%).

How certain are we?

GRADE Moderate certainty.


We recommend that trialists consider enclosing a questionnaire covering issues relevant to the trial with the trial invitation.

How can I use this straight away?

See Resource bundle below for details on how to prepare a questionnaire covering issues relevant to the 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 adding a questionnaire covering issues relevant to the trial to the invitation pack. The chart below shows the impact of an absolute increase of 18% (95% CI = 16% to 20%). Recruitment is now 48%, which means our best estimate is that 63 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 moderate because of the imprecision of a single study.

Resource Bundle

How to Cite

Citation: Treweek S, Bruhn H, Gardner H. Evidence pack­– Recruitment: Questionnaire covering issues relevant to the trial (REC11), 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 large effect size and GRADE Moderate 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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