Twenty-two RCTs (n=1,091) were included in the review and 21 in the meta-analyses.
The authors stated that most studies used outcome measures that were of unknown reliability and validity. The mean drop-out rate was 12.2%.
Hypnotherapy was associated with a statistically significant benefit compared with control (d=0.61, defined as medium; p=0.000). Significant heterogeneity was found (p=0.00024). An additional 56 studies would be required to reduce this effect size to 0.20 (fail-safe N). The funnel plot suggested the absence of larger studies with larger effect sizes.
The cluster analysis identified three clusters at the 5% significance level which were statistically homogeneous (studies in each cluster were listed).
The weighted analysis of variance found that only the type of hypnotherapy was associated with a significant effect on effect size. Effect sizes were higher for mixed and modern hypnotherapy compared with classical hypnotherapy (d=1.42 for modern, defined as large and d=0.73, defined as medium, for mixed; and d=0.33, defined as medium, for classical).
Three of the 7 studies assessing the correlation between suggestibility and effect size reported a significant positive correlation. Of the remaining 4 studies, three reported a positive correlation and one did not find any correlation.
There was no significant relationship between effect size and number of treatment sessions (p=0.093).
Data on pre- and post-treatment comparisons were also reported.