Making trials less lossy: is there anything worth knowing from non-randomised evaluations of trial retention strategies?
Brighton, UK

Speaker: Adel El Feky

This is a talk at the International Clinical Trials Methodology Conference (ICTMC), Brighton, UK, 6th – 9th October 2019.  Venue is the Hilton Brighton Metropole.

Parallel Session 6A – Retention to Trials


Poor retention is common. It reduces statistical power and can bias the estimates of intervention effect, especially with differential loss-to-follow across trial arms. Given the sparsity of evidence from randomised evaluations of effective retention strategies, we performed a systematic review to synthesise evidence from non-randomised evaluations to supplement existing evidence.


We searched MEDLINE, Embase, and Cochrane CENTRAL from 2007 to 2017 for studies that compared two or more strategies to increase participant retention in randomised trials, but did not use randomisation for allocation. The retention trials had to be nested in real ‘host’ trials.  

Abstract and text screening was done in duplicate. Two investigators independently rated the risk of bias of included studies using the ROBINS-I tool and determined the certainty of evidence using GRADE.



We identified 7609 abstracts and included 14 studies in the review. Most retention strategies were targeted at increasing questionnaire response rate rather than face-to-face appointments. Six strategies suggested a promising increase in questionnaire response rates: telephone follow-up compared to postal questionnaire completion; electronically-transferred monetary incentives compared to cash incentives; cash incentive versus no incentive; reminders to non-responders; shortened versus longer questionnaires; online questionnaire follow-up compared to postal questionnaire [absolute increase in retention ranged from 10-40%]. However, each retention strategy was evaluated in a single observational study, which together with risk of bias concerns meant that the overall GRADE certainty was low or very low for all included studies.


This systematic review provides low or very low certainty evidence on the effectiveness of retention strategies evaluated in non-randomised studies. Despite the uncertainty, some of the reported effect sizes were substantial and would remain large even if greatly reduced. Further evaluation in randomised studies (particularly telephone follow-up) would be helpful to provide a more certainty around the actual effect size.

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