Die Effektivität der Hypnose

From 193 studies published until 1998 that investigated the efficacy of hypnosis 43 randomized clinical studies were selected that compared a patient group treated exclusively by hypnosis with an untreated control group (or with a group of patients treated by conventional medical procedures). The 43 studies were integrated into a meta-analysis that yielded a weighted average post-treatment effect size of d=.60 (medium effect size) for hypnotic treatment of ICD-10 coded disorders (24 studies; average treatment period: 5.1 weeks) and d=.38 (small effect) for hypnosis as an adjunct for supporting medical procedures (19 studies). These estimates are conservative since all variables of a given study were used. Most of the studies employed methods of the classical approach to hypnosis. In order to obtain an estimate to which extent non-clinical factors (design-quality, way of comparison of dependent variables) have an influence on the effect sizes, effect sizes were computed for all studies of the original 193 studies that reported the necessary statistical information (N=89). For those studies with an average effect size of d=.80 a massive influence of non-clinical factors was demonstrated with a range from d=.51 for randomized studies with group comparisons to d=2.0 for non-randomized studies using pre-post-comparisons.