Study within a trial 119: The effectiveness of a thank you card to improve trial follow up; a randomised study within a trial

Abstract

Background With attrition common in randomised trials, strategies are needed to minimise this. Many retention strategies include ‘thanks’ elements however there is currently no evidence of the effectiveness of a ‘thank you’ intervention separate to other trial activity or information. This Study Within A Trial (SWAT) sought to assess if a thank you card increases completion of the host trial primary outcome. Methods A two arm SWAT, using a 1:1 (intervention:control) allocation ratio, embedded within the DISC trial. The primary outcome was the difference in retention rate at 1 year post-treatment. Secondary outcomes were outcome data completeness, cost, and retention at 2 years post-treatment. Analyses were conducted using logistic regression adjusting for SWAT and host trial allocation. Results A total of 358 participants were randomised and included in the SWAT analyses. Completion of the 1-year outcome visit was 89.7% ( n = 157) in the intervention group and 90.2% (165) in the control group (adjusted odds ratio (OR) 0.95, 95% CI 0.48 to 1.90, p = .89). There was no evidence of a difference in completeness of key outcome data (adjusted OR 1.84, 95% CI 0.71 to 4.73, p = .20) or retention at 2 years post treatment (adjusted OR 1.13, 95% CI 0.59 to 2.17, p = .72). Conclusion It remains unclear if thank you cards increased the rate of primary outcome follow-up completion within the DISC trial. However, as the first evaluation of a distinct ‘thank you’ intervention for improving retention rates, further replications are required to determine effectiveness, ideally in populations other than older, male, Caucasians.