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Financial incentives with prompt or reminder (ID RET4)

Evidence Summary

What is it?

Participants are offered a conditional financial incentive (e.g., £10 gift voucher posted out on receipt of a completed questionnaire) either on the first questionnaire mail-out or on reminder letters only.

Does it work?

Addition of a monetary reward to both trial arms delivered either with the prenotification or with the reminder letter, probably increases retention.

How big is the effect?

An increase of 9% (95% confidence interval = 3% to 15%).

How certain are we?

GRADE Moderate certainty.


We suggest that trialists consider using financial incentives, ideally as part of a SWAT evaluation.

How can I use this straight away?

See Resource bundle below for details of how to use financial incentives or build a SWAT evaluation of them into your trial.

Practical Impact

Imagine initial retention is 65%. 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).

Now imagine using financial incentives with the prompt or reminder. The chart below shows the impact of an absolute increase of 9% (95% CI = 3% to 15%). Retention is now 74%, which means our best estimate is that you would now only be 6 responses short.

Cumulative Meta-Analysis*

*Random effects model done using Comprehensive Meta-Analysis v4 ( Differences >0% favour the intervention. The GRADE assessment is moderate because of imprecision due to being a single study.

Resource Bundle

How to Cite

Citation: Treweek S, Bruhn H. Evidence pack– Retention: Financial incentives as a way to avoid reminders for questionnaire return (Ret4), 2021

More Information

  1. This summary is from the Cochrane review of strategies to improve retention in randomised trials (
  2. The summary 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 moderate effect size and Moderate GRADE 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 - 05/07/2023
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