Principal Findings
SECONDARY
This study sought to examine whether the ItFits-toolkit leads to better implementation outcomes than IAU in implementing iCBT services. In comparison with IAU, the ItFits-toolkit has a small statistically significant effect on normalization levels in iCBT service providers. The toolkit did not have an effect on iCBT service uptake by patients, and the implementers did not spend more time using the toolkit. ItFits-toolkit users regarded the toolkit generally as usable and were satisfied with it. These findings fit the general pattern across tailoring studies [To the best of our knowledge, this was the first study to investigate the effectiveness of self-guided tailored implementation of eMental health services supported by a web-based implementation toolkit using a standardized, validated, and quantitative primary implementation outcome in service delivery staff in mental health settings. Practically, these findings can contribute to implementing iCBT services into routine care by delivering a functional and technically stable toolkit. The toolkit was easy to use, provided implementers with flexible ways to structure and infuse their work with scientific knowledge, and involved relevant stakeholders in developing and executing tailored implementation strategies. Findings indicated that implementation can be improved with a self-guided toolkit that enhanced implementation outcomes without extra investments of effort. Despite the small effect and the need for further research to better understand and optimize outcomes, clinical directors, managers, and implementers may consider using the current version of the toolkit for implementing iCBT services. The toolkit is freely accessible and may improve the outcome of local staff-driven implementation activities.A temporal effect became apparent, pointing toward a small decline in normalization scores (total and of the underlying 4 constructs) in the IAU condition. The ItFits-toolkit partially canceled out this negative trend. This is surprising as, following the principles of NPT, we expected the normalization scores to increase when implementers and service providers engaged and worked to implement the iCBT services. Although speculative, this decline in normalization may be related to the complexity of iCBT services for service providers to deliver iCBT to their patients. Approximately three-quarters of the service deliverers were inexperienced in delivering iCBT before the study. As service providers started to spend more time delivering iCBT services, they might have gained a better understanding of its complexity and changes required to successfully integrate and embed the service in their routine practice, resulting in a decline in normalization scores. Similarly, gaining experience with providing iCBT services while at the same time being confronted with considerable patient attrition rates might have impacted the perception of service providers toward implementing the iCBT services. Furthermore, the possibly waning enthusiasm of service providers and implementers toward the research conducted and being part of a large-scale international research project might have influenced implementation outcomes. More research and debate are required to fully understand the theoretical implications of these findings.For the secondary outcome, and although there are differences between organizations and the toolkit had no effect on service uptake, the high treatment dropout rate (72.6%) should be noted. The debate on dropout, adherence, and treatment completion was unsettled, and definitions differed greatly. A systematic review of dropout rates in research trials investigating the effectiveness of guided and unguided iCBT ranges from 0% to 78%, with an average of 21% stopping treatment early [