Poor retention of participants in randomised trials can lead to missing outcome data which can introduce bias and reduce study power, affecting the generalisability, validity and reliability of results. Many strategies are used to improve retention but few have been formally evaluated. This review aimed to quantify the effect of strategies to improve retention of participants in randomised trials and to investigate if the effect varied by trial setting.

We included eligible randomised or quasi‐randomised trials of evaluations of strategies to increase retention that were embedded in 'host' randomised trials from all disease areas and healthcare settings. We excluded studies aiming to increase treatment compliance.

Most of the interventions we identified aimed to improve retention in the form of postal questionnaire response. There were few evaluations of ways to improve participants returning to trial sites for trial follow‐up. None of the comparisons are supported by high‐certainty evidence. Comparisons in the review where the evidence certainty could be improved with the addition of well‐done studies should be the focus for future evaluations.


Katie Gillies;