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Social incentive (ID RET20)

Evidence Summary

What is it?

social incentive such as a personalised table of questionnaire response to date to evidence previous responses noted and valued are included in the cover letter.

Does it work?

Including a social incentive in the cover letter may result in little or no difference to retention.

How big is the effect?

An increase of -1% (95% confidence interval = -4% to 2%).

How certain are we?

GRADE Low certainty.


We recommend that trialists include a social incentive in cover letters in the context of an intervention evaluation.

How can I use this straight away?

See Resource bundle below for details on how to use social incentives.

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 a social incentive. The chart below shows the impact of an absolute increase of -1% (95% CI = -4% to 2%). Retention is now 64%, which means our best estimate is that you would now be 14 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 and a wide CI crossing RD=0.

Resource Bundle

How to Cite

Citation: Ostrovska B. Evidence pack– Retention: Social incentive (RET20), 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 trivial 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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