The research questions we investigated in this study were twofold. Our first hypothesis addressed the overall impact of an Internet based cognitive behavioural intervention (Interapy) on a sample of patients with PTSD or subsyndromal PTSD. We found significant statistical and clinical effects that indicated symptom reduction of PTSD in the treatment group. Furthermore, a reduction in psychological symptoms related to depression, anxiety and mental health accompanied improvements in PTSD symptoms. However, the participants with trauma-related symptoms and depression in the control group also improved significantly on trauma-related symptoms and depression. Furthermore, results indicate that treatment gains were maintained up to 3 months after the completion of treatment. This is in line with previous studies of internet-driven CBT for posttraumatic stress reactions , complicated grief  and CBT interventions in face-to-face studies [10, 31]. This was the first cross-culturally applied study examining Interapy in a German speaking sample. It replicated the findings of Lange et al.  and validates this treatment approach by indicating effectiveness, acceptability and the applicability across different countries. Although, several effective treatment approaches for PTSD have been available for a considerable time, accessibility remains a problem due to difficulties in establishing and maintaining effective methods of dissemination of these treatment methods to treatment providers . In the Netherlands, Interapy is already integrated into the regular health care system and is accessible nationwide. But since the assessment is exclusively based on questionnaires no formal diagnosis has been able to be established over the Internet. In face-to-face interactions the assessment is carried out by trained psychologists during an interactive diagnostic process. Assessment models should be developed to be implemented over the Internet. Thus, further evidence is needed before conclusions about the generalizability for a general population of PTSD patients can be drawn. Future research should directly compare face-to-face with Internet based intervention after establishing a clinical diagnosis face-to-face to be able to evaluate the efficacy of Internet based therapy more clearly.
Furthermore, we were interested in finding out whether a positive and stable relationship can be maintained online, whether the therapeutic alliance would improve throughout treatment and whether the quality of the online therapeutic relationship would have a moderating effect on treatment outcome. High ratings of the working alliance (at the end of treatment: patients M = 6.3; therapists M = 5.8 on a scale from 1–7) of both parties were obtained. Callahan, Price, and Hilsenroth  assessed the working alliance in face-to-face therapy with the WAI at the end of treatment. They found mean alliance ratings of M = 5.5 (child abuse survivors) and M = 5.4 for patients with other psychiatric disorders. Surprisingly, the bond-dimension of the working alliance which comprised statements such as: "Me and my therapist trust each other" was rated particularly high in our study even at an early stage of treatment (4th session). Also, a relatively low drop-out rate (16%) and the fact that the majority rated this exclusively internet-based contact as positive (76%) and personal (86%) indicated stable and positive therapeutic relationship online. Significant improvement of the therapeutic relationship rated by patients could be observed during the course of treatment. Findings on face-to-face studies identified three typical patterns: a stable alliance pattern, a linear growth pattern and a u-shaped pattern . Possibly, the alliance formation observed in this study is similar to the development of the therapeutic relationship in face-to-face therapies. Alternatively, it might also be the case that the therapeutic alliance online, particularly in the eyes of the patients, may not have stabilized by the fourth writing session. This would be in line with Walther  who found that the difference in quality between online and face-to-face relationships is moderated by the duration of the relationship and the frequency of contact. In other words, the degree of intimacy is influenced by the amount of information that is exchanged. Repeated assessment of the working alliance and an immediate comparison with a face-to-face intervention would be needed to find out whether this would also apply to online therapeutic relationships. Therapists' alliance rating showed no variation.
According to our hypothesis we found a substantial correlation between the late therapeutic alliance and treatment outcome. This is in line with previous findings of face-to-face studies of CBT showing that substantial amounts of outcome variance were uniquely accounted for by alliance scores . However, an alternative explanation for the correlation between working alliance and treatment outcome might be that ratings of the quality of the working alliance might have been confounded with outcome. Thus, instead of being a predictor for outcome the rating of the alliance would be an additional indirect measure of outcome. Previous analysis of the online working alliance early in treatment revealed no substantial correlation between the working alliance and treatment outcome . Further research is needed to understand the therapeutic contribution of the online therapeutic alliance. Measurement of the working alliance and symptom level at several points during the whole therapeutic process would help to understand the relation between online therapeutic alliance and outcome.
In the current study, we sought to ascertain the efficacy of an internet-driven treatment for PTSD and the quality and the role played by the online therapeutic alliance. The examination of an online therapeutic alliance is of particular relevance since it has proven to be a stable predictor in face-to-face therapy.
Among the limitations of this study is the screening strategy for the recruitment of the patients. We deliberately handled strict exclusion criteria for participation in this study. We excluded 72% (n = 253) of the patients who wanted treatment but did not meet the inclusion criteria. This might limit the generalizability of our results. Also, the sample was mainly female, better educated and younger than the general population. Another methodological concern might be the choice of the questionnaire. We used the frequently applied Working Alliance Inventory because of its pantheoretical nature which allowed its use in many different treatment approaches. However, the WAI was not designed for an internet-driven type of therapy and it might be that it is a less valid instrument for capturing an online therapeutic alliance. A further limitation is that we included a waiting list control group instead of placebo control group. This design will likely result in higher effect sizes compared to a placebo control group. In addition, as we employed a waiting list controlled design, it would have been unethical to deny treatment to those patients originally randomized to the waiting list. Consequently, there is no control group against which the outcomes at the follow up assessments of the treated sample can be compared. This limits the evaluation of long-term effects of this intervention. Finally, treatment outcomes were measured mainly by self-rated questionnaires administered through the Internet only. Interviews or other independent assessments would have added to the validity and clinical value of the results.
Although the results of the present study are promising, there is a need for further studies concerning the applicability and efficacy of online therapy and specific underlying processes such as the development of the therapeutic alliance and its distinctive cross-method features. Further analysis of the 18 months follow-up data and the examination of other potentially relevant moderators such as posttraumatic growth [Maercker & Knaevelsrud, in preparation] will hopefully enhance our understanding of online therapeutic processes. Considering that online therapy is gaining acceptance  and provides a cost-efficient, worldwide accessible alternative it is imperative that we increase our understanding of this new treatment approach.