Evidence Summary
What is it?
Modified cover letter sent with trial questionnaires. The letter is created as a behavioural change intervention using psychological theory, the behaviour to change being return of the questionnaire.
Does it work?
Sending a theory-based cover letter probably increases retention.
How big is the effect?
An increase of 4% (95% confidence interval = -1% to 10%).
How certain are we?
GRADE Moderate certainty.
Recommendation
We recommend that trialists use theory-based cover letters to increase retention in trials that use questionnaires.
How can I use this straight away?
See Resource bundle below for the theory-based cover letter template and other support materials
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 theory-based cover letters. The chart below shows the impact of an absolute increase of 4% (95% CI = -1% to 10%). Retention is now 69%, which means our best estimate is that you would now only be 11 responses short.
Cumulative Meta-Analysis*
*Random effects model done using Comprehensive Meta-Analysis v3 (www.meta-analysis.com).
Differences >0% favour the intervention. The GRADE assessment is moderate because of imprecision.
Resource Bundle
How to Cite
Citation: Treweek S, Gillies K. Evidence pack– Retention: Theory-based cover letter (Ret1), 2020, https://www.trialforge.org/retention-sector/theory-based-cover-letter/.
More Information
- This summary is from the Cochrane review of strategies to improve retention in randomised trials (https://www.cochranelibrary.com/cdsr/doi/10.1002/14651858.MR000032.pub3/full).
- 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 moderate effect size and High GRADE certainty
- The IQuaD and INTERVAL data are from Goulao et al 2020 (https://doi.org/10.1016/j.jclinepi.2020.01.018). AMBER and OPAL are presented in Duncan et al 2019 (PS1A– https://doi.org/10.1186/s13063-019-3688-6).
- 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.
- If you have any questions contact info@trialforge.org.
