Strengths and Limitations
To the best of our knowledge, this randomised controlled trial will be the first to test the effects of a web-based CBT self-help course for people with diabetes and co-morbid depression. The intervention builds on an existing course that has shown to be feasible and effective to help improve mood in the general population . Our trial is sufficiently powered to allow for conclusions as to its short-term effectiveness (6 months). In view of the "lenient" inclusion and exclusion criteria we will apply, the study is expected to have high external validity.
An obvious limitation of this trial is the fact that only people with access to the Internet and sufficient computer skills can be included. Overall, an estimated 82% of the Dutch households are online in 2007, but access to Internet among the elderly is much lower  and as a consequence this group may therefore be under represented in our study.
Inclusion and exclusion criteria are clearly stated on the website. This has the potential disadvantage that people can pretend to have certain characteristics in order to qualify or they can deny having "exclusion criteria". However, this response tendency may be diminished by the fact that patients are being made aware that we inform their GP's of their participation in the study and that we will be in contact with their treating physician concerning their HbA1c results.
As the post-treatment assessment will be done directly after treatment and up till 6 months follow-up, no conclusions can be drawn about the long-term effects of the intervention. Therefore, after 6 months, patients will be asked to give permission for supplemental follow-up measurements at one and two years after completion of the course. These results will be analysed at a later stage and are not part of this project.
Although web-based administration of questionnaires is widely used, we should remind ourselves that the psychometric properties of internet-administered questionnaires may not be equal to those of paper-and-pencil versions . On the other hand, in an earlier study we have found that the paper-and-pencil and the computerised versions of a short psychological well-being questionnaire appeared to be equivalent . The major difference appears to be that respondents are more honest and revealing when filling out via the Internet, resulting in a relatively higher reported level of symptomatology. We have therefore chosen to confirm or decline the diagnosis of depression by a telephone administered diagnostic interview (CIDI). Our study will show if such diagnostic procedure is required in future projects applying W-CBT in order to reduce the risk of 'false positives'.
Also the psychometric properties of the following on-line questionnaires will be examined and compared to the paper-and-pencil versions: the PAID, SF-12 and the ADSCI.
We hypothesise that improved mood will be associated with improved diabetes self-management and we will test whether W-CBT has favourable effects on self-reported diabetes self-care behaviours, using the Amsterdam Diabetes Self-care Inventory (ADSCI) that was developed by our team and used in previous trials [12, 13, 34]. The psychometric properties of the Amsterdam Diabetes Self Care Inventory (ADSCI) have not yet been published, but a report on the validity and reliability of the scale is planned based on data on file.
Ethical and practical considerations have led us to decide that all participants are allowed to use additional mental health care (MHC) services during the trial. Usage of additional services therefore is a potential confounder that will be measured and taken into account in the statistical analyses.
Drop-out is often a problem in web-based therapy trials. In adapting the generic coping with depression course to the needs of people with diabetes, we expect to reduce drop-out. In an attempt to further reduce the attrition rate, support e-mails will be send out to participants by their coach, to encourage them to stay involved in the course. Likewise, those in the waiting list condition will receive e-mails aimed to maintain contact and encourage them to participate.
The study protocol was approved by the VU University Medical Centre ethics committee, which is certified by the Central Committee on Research involving Human Subjects in the Netherlands.