BACKGROUND: Poor retention in randomised trials can lead to serious consequences to their validity. Studies within trials (SWATs) are used to identify the most effective interventions to increase retention. Many interventions could be applied at any follow-up time point, but SWATs commonly assess interventions at a single time point, which can reduce efficiency.
METHODS: The re-randomisation design allows participants to be re-enrolled and re-randomised whenever a new retention opportunity occurs (i.e. a new follow-up time point where the intervention could be applied). The main advantages are as follows: (a) it allows the estimation of an average effect across time points, thus increasing generalisability; (b) it can be more efficient than a parallel arm trial due to increased sample size; and (c) it allows subgroup analyses to estimate effectiveness at different time points. We present a case study where the re-randomisation design is used in a SWAT.
RESULTS: In our case study, the host trial is a dental trial with two available follow-up points. The Sticker SWAT tests whether adding the trial logo's sticker to the questionnaire's envelope will result in a higher response rate compared with not adding the sticker. The primary outcome is the response rate to postal questionnaires. The re-randomisation design could double the available sample size compared to a parallel arm trial, resulting in the ability to detect an effect size around 28% smaller.
CONCLUSION: The re-randomisation design can increase the efficiency and generalisability of SWATs for trials with multiple follow-up time points.
We would like to thank the trial teams involved in delivering the Sticker SWAT, including the clinical Chief Investigators of REFLECT (Profs Martin Tickle and Jan Clarkson) and of C-GALL, another NIHR-funded trial testing the same retention intervention, Prof Irfan Ahmed. We thank Prof David French and Prof Marie Johnston for their valuable insights regarding the literature of repeated behaviour change interventions.
REFLECT is funded by the NIHR HTA Programme (project number 16/23/01). BG has been supported by the Wellcome Trust Institutional Strategic Support Fund at the University of Aberdeen. The Health Services Research Unit is funded by the Chief Scientist Office of the Scottish Government Health and Social Care Directorates. The views expressed are those of the author(s) and not necessarily those of the NHS, the NIHR or the Department of Health. B.C.K. is funded by the UK MRC, grants MC_UU_00004/07 and MC_UU_00004/09.
- Trial methodology
- SWAT (study within a trial)