Non-randomised evaluations of strategies to increase participant retention in randomised controlled trials: a...

Retention of participants is essential to ensure the statistical power and internal validity of clinical trials. Poor participant retention reduces power and can bias the estimates of intervention effect. There is sparse evidence from randomised comparisons of effective strategies to retain participants in randomised trials. Currently, non-randomised evaluations of trial retention interventions embedded in host clinical trials are rejected from the Cochrane review of strategies to improve retention because it only included randomised evaluations. However, the systematic assessment of non-randomised evaluations may inform trialists' decision-making about retention methods that have been evaluated in a trial context.Therefore, we performed a systematic review to synthesise evidence from non-randomised evaluations of retention strategies in order to supplement existing randomised trial evidence.

We searched MEDLINE, EMBASE, and Cochrane CENTRAL from 2007 to October 2017. Two reviewers independently screened abstracts and full-text articles for non-randomised studies that compared two or more strategies to increase participant retention in randomised trials. The retention trials had to be nested in real 'host' trials ( including feasibility studies) but not hypothetical trials. Two investigators independently rated the risk of bias of included studies using the ROBINS-I tool and determined the certainty of evidence using GRADE (Grading of Recommendations Assessment, Development and Evaluation) framework.

Fourteen non-randomised studies of retention were included in this review. Most retention strategies (in 10 studies) aimed to increase questionnaire response rate. Favourable strategies for increasing questionnaire response rate were telephone follow-up compared to postal questionnaire completion, online questionnaire follow-up compared to postal questionnaire, shortened version of questionnaires versus longer questionnaires, electronically transferred monetary incentives compared to cash incentives, cash compared with no incentive and reminders to non-responders (telephone or text messaging). However, each retention strategy was evaluated in a single observational study. This, 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. Some strategies need further evaluation to provide confidence around the size and direction of the underlying effect.