Relation of study design to recruitment and retention in CTN trials.

Background: Recruitment and retention in randomized clinical trials are difficult in general and particularly so in trials of substance abuse treatments. Understanding trial design characteristics that could affect recruitment and retention rates would help in the design of future trials.
Objective: To test whether any of the following factors are associated with recruitment or retention: type of intervention, type of therapy, duration of treatment, total duration of trial, number of treatment sessions, number of follow-up visits, number of primary assessments, timing of primary assessments, number of case report form (CRF) pages at baseline, and number of CRF pages for the entire trial.
Methods: Recruitment and retention data from 24 Clinical Trials Network (CTN) trials conducted and completed between 2001 and 2010 were analyzed using single-factor analysis of variance and single-predictor regression methods to test their association with trial design characteristics.
Results: Almost all of the analyses performed did not show statistically significant patterns between recruitment and retention rates and the trial design characteristics considered.
Conclusion: In CTN trials, the relationship between assessment burden on participants and length of trial, on the one hand, and recruitment and retention, on the other, is not as strong and direct as expected. Other factors must impinge on the conduct of the trial to influence trial participation.
Scientific Significance: Researchers may deem slightly more justifiable to permit inclusion of some of the design features that previously were assumed to have a strong, negative influence on recruitment and retention, and should consider other strategies that may have a stronger, more direct effect on trial participation.