Attrition and adherence in smartphone-delivered interventions for mental health problems: a systematic and meta-analytic review

Objectives: Although the efficacy of smartphone-delivered interventions for mental health problems is emerging, randomized controlled trials (RCTs) of smartphone interventions are characterized by high rates of attrition and low adherence. High attrition and low adherence may threaten the validity of RCT findings, so a better understanding of these phenomena is needed. We examined attrition and adherence in 70 RCTs of smartphone interventions. Method: Four online databases were searched for RCTs of mental health interventions delivered via smartphones. Results: The mean meta-analytic study attrition rate was 24.1% (95% CI [19.3, 29.6]) at short-term follow up and 35.5% (95% CI [26.7, 45.3]) at longer-term follow up. These rates varied according to target mental health condition. Attrition rates were significantly lower in trials that delivered an acceptance-based intervention, offered participants monetary compensation, and reminded participants to engage in the intervention, and were significantly higher in trials that used an online enrollment method (relative to telephone or in-person enrollment). No participant-level baseline characteristic reliably predicted attrition. Evidence of attrition bias came from many RCTs not conducting intention-to-treat analyses. However, the mean difference in the between-groups effect size on primary outcomes in trials that reported both per protocol an intention-to-treat analyses was only Δd = 0.18. Adherence rates were also suboptimal based on our qualitative synthesis; several participants failed to download the intervention, and intervention usage consistently declined over the course of the trial. Conclusion: Study attrition and low adherence are common, problematic, and may undermine the validity of findings in RCTs of smartphone-delivered interventions for mental health problems.