On the efficacy of hypnosis: A meta-analytic study

American Journal of Clinical Hypnosis,  Jul 2004  by Hammond, D Corydon

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Flammer, E. & Bongartz, W. (2003). On the efficacy of hypnosis: A meta-analytic study. Contemporary Hypnosis, 20(4), 179-197. The authors reviewed 444 studies on the efficacy of hypnosis published before 2002. Fifty-seven randomized studies were selected where there was an untreated control group or alternative treatment (e.g., traditional medical treatment) group.

A meta-analysis was done yielding a weighted average posttreatment effect size of .56 (medium effect size), but of .63 for the hypnotic treatment of ICD-IO diagnosed disorders. However, "these estimates are conservative since all variables of a given study were used" (p. 179). In an effort to estimate the influence of nonclinical factors (such as quality of research design, way of comparing dependent variables) effect sizes were computed for all 444 studies, with an average effect size of 1.07. They concluded that nonclinical factors exerted a massive influence ranging from .56 for randomized studies with group comparisons to 2.29 for nonrandomized studies using only pre-post comparisons. Only 6 of 57 studies reported correlations between hypnotic suggestibility level and treatment outcome, and the mean correlation was .44. Address for reprints: Prof. Walter Bongartz, Universität Konstanz, Math.-Naturwiss. Sektion, Fachbereich Psychologie, 78457 Konstanz, Germany. Email: walter.bongartz@uni-konstanz.de.

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