Statistical analysis
Effect size
Standardized mean pre- to post- psychotherapy changes were evaluated. Effect side (cohen’s d) were calculated by subtracting post-treatment scores form pre-treatment scores and dividing these by the pooled standard deviations. To calculate the pooled s.d. the test-retest reliability for the most common instrument in that domain was taken into account. Follow-up effect size were calculated by subtracting follow-up scores form post-treatment scores and dividing by the pooled s.d. to determine the sustain-ability of effect. We selected the longest time frame between treatment termination and follow-up. Effect size of 0.20, 0.50 and 0.80 are considered small, medium and large respectively; effect size below 0.20 are considered very small. Effect sizes were weighted by their inverse variance. This procedure gives more weight to larger studies (with smaller sampling error), thus increasing the reliability of the effect estimates.
Within the same study, effect sizes were calculated separately for one or more treatment as usual (TAU) control condition. To be regarded as TAU, treatments had to be specified by the researchers as such or as (standard or enhanced) medical care, psychiatric advice to the general practitioner or TAU with a small component added (e.g. relaxation or gymnastics). Waiting-list control group were not considered to be TAU. When the alleged comparison condition clearly included psychotherapy as a main component, the condition was categorised as psychotherapy