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Open trial design (ID RET13)

Evidence Summary

What is it?

As opposed to a blinded placebo-controlled design, participants receive information based on the open trial design.

Does it work?

Randomising participants at recruitment to an open rather than a blinded trial may result in a large increase in retention.

How big is the effect?

An increase of 13% (95% confidence interval = 4% to 22%).

How certain are we?

GRADE Low certainty.


We recommend that trialists consider using an open trial design where possible.

How can I use this straight away?

See Resource bundle below for details on how to use an open trial design.

Practical Impact

Imagine initial retention is 65% of those approached. You have a trial with 100 participants that needs responses from 80 to meet its statistical power calculations.  Retention of 65% means that you will be 15 responses short (see chart below).

Now imagine using open trial design. The chart below shows the impact of an absolute increase of 13% (95% CI = 4% to 22%). Retention is now 78%, which means our best estimate is that you would now only be 2 responses short.

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.

Resource Bundle

How to Cite

Citation: Ostrovska B. Evidence pack– Retention: Open trial design (RET13), 2023,

More Information

  1. This summary is from the Cochrane review of strategies to improve retention in randomised trials (
  2. The ‘Does it work?’ statement is structured according to effect size and GRADE certainty as per GRADE Guidelines 26 ( The statement is for large effect size and Low GRADE certainty.
  3. 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.
  4. If you have any questions contact
v1.0 - 06/07/2023
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