Sending a pre-recruitment primer letter

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


The absolute increase is 0% (95% CI = -6% to 6%)1

How confident are we in the evidence?  
What is GRADE?

LOW CERTAINTY in the evidence (but see 'What we don't know' below)

1 Treweek S, Pitkethly M, Cook J, Fraser C, Mitchell E, Sullivan F, Jackson C, Taskila TK, Gardner H. Strategies to improve recruitment to randomised trials. Cochrane Database of Systematic Reviews 2018, Issue 2.

The practical impact of sending a pre-recruitment primer letter

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 (see chart below).

No primer letter
No primer letter

Now imagine sending a pre-recruitment primer letter. The chart below shows the impact of an absolute increase of 0% (95% CI = -6% to 6%)1. Recruitment is still 30%, which means our best estimate is that 100 people would still need to be approached to recruit 30 of them.

Sending primer letter
Sending primer letter
By sending a pre-recruitment primer letter, our best estimate is that you will need to approach the same number of participants as without the primer.

Where has sending a pre-recruitment primer letter been tested?


Sending a pre-recruitment primer letter


No primer letter


1 study including a total of 1062 participants

Study 1: Paul 2014

Participants were 18 years or older with a primary colorectal cancer diagnosis and within 3 months of diagnosis and on registry.

Trial intervention?
Unclear but recruitment is to a colorectal cancer screening trial.

Study location?
Community, Australia.

What difference?
39.9% of participants randomised to receive a primer letter were recruited; 40.1% of participants randomised to not receive a primer letter were recruited

Read more about this study here.

What we still don’t know about sending pre-recruitment primer letters

  • The GRADE assessment is low for this intervention because the there is only a single evaluation with imprecision. More evaluations in any type of trial are needed.
  • Please get in touch (email if you would like to do an evaluation because we can help with text for ethics etc.

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