Internet-based psychotherapeutic interventions have been used for over a decade, but no comprehensive review and no extensive meta-analysis of their effectiveness have been conducted. We have collected all the empirical articles published up to March 2006 (n=64) that examine the effectiveness of online therapy of different forms and performed a meta-analysis of all the studies reported in them (n=92). These studies involved a total of 9,764 clients who were treated through various Internet-based psychological interventions for a variety of problems, whose effectiveness assessed by different types of measures. The overall mean weighted effect size was found to be 0.53 (medium effect), which is quite similar to the average effect size of traditional, face-to-face therapy. Next, we examined interacting effects of various possible relevant moderators of the effects of online therapy, including type of therapy (self-help Web-based therapy vs. online communication-based eTherapy), type of outcome measure, time of measurement of outcome (post-therapy or follow-up), type of problem treated, therapeutic approach, and communication modality, among others. A comparison between face-to-face and Internet intervention as reported on in 14 of the studies revealed no differences in effectiveness. The findings of this meta-analysis, and review of additional Internet therapy studies not included in the meta-analysis, provide strong support for the adoption of online psychological interventions as a legitimate therapeutic activity and suggest several insights in regard to its application. Limitations of the findings and recommendations concerning Internet-based therapy and future research are discussed.
