Evidence Summary

What is it?

Participants are mailed an explanatory primer letter designed to encourage research participation (highlights the importance of participation in research studies, the benefits to the individual an to the community among others).

Does it work?

Sending a recruitment primer letter may have little or no effect on recruitment.

How big is the effect?

An increase of 0% (95% confidence interval = -6% to 6%).

How certain are we?

GRADE Low certainty.


We recommend that trialists send a pre-recruitment primer letter in the context of an intervention evaluation.

How can I use this straight away?

See Resource bundle below for details on how to write pre-recruitment primer letters.

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 sending a pre-recruitment primer letter. The chart below shows the impact of an absolute increase of 0% (95% CI = -6% to 6%). Recruitment is still 30%, which means our best estimate is that 100 people would still need to be approached to recruit 30 of them.

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 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: Pre-recruitment primer letter (REC16), 2023, https://www.trialforge.org/recruitment-sector/pre-recruitment-primer-letter-id-rec16

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. 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 (https://doi.org/10.1016/j.jclinepi.2019.10.014). The statement is for trivial, small unimportant effect or no effect 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 info@trialforge.org.
Version: v2.0 - 10/07/2023

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