Ninety-two studies were included in the review (n=9,764, range six to 2,341).
The overall mean weighted effect size was 0.53; average effects varied from a minimum effect size of –0.10 (treating panic disorder through online information alone) to a maximum of 1.68 (treating smoking cessation through tailored CBT). Older adults (25 to 39 years) formed the age category that was treated most effectively (ES 0.62; n=6,941).
Post-traumatic stress disorder (ES 0.88; n=148) and panic and anxiety disorders (ES 0.80; n=498) were the types of problem that were most effectively treated by online therapy; significant heterogeneity was present. Compared with other therapeutic approaches applied online, CBT (ES 0.83; n=3,960) was significantly more effective; behavioural techniques appeared to be the least suited for online treatment (ES 0.23; n=1,136). Significant heterogeneity was present. Web-based therapy (ES 0.54) was found to be as effective as etherapy (ES 0.46).