Chapter 2 – Meta-analysis 37 and others reported nocebo magnitudes as the mean difference of all control and evocation trials. When only nocebo/control difference scores were reported, these were used instead. When only standard errors were reported, they were converted to standard deviations by multiplying the standard error by the square root of the group size (n). For each study, an effect size Hedges’s g, weighted to the sample size (N), was computed as the mean response in the nocebo condition minus the mean response for control in the evocation phase of experiments. Positive g values indicate a nocebo response, with values around .2 considered small, .5 medium, and .8 large. For studies that used within-subjects comparisons, the nocebo-control condition correlation coefficient could not be derived, therefore an average r of .5 was imputed 28. Meta-analysis was only conducted when the data of at least 4 studies were available in total. In subset analyses, the effect sizes were compared descriptively rather than with statistical tests when 2 or less studies were available per group. Studies with multiple eligible conditions were treated as separate subgroups and averaged across in CMA (e.g., when cheap vs. expensive inert treatments were used as nocebo, we averaged the results and treated this as one group (see Table 1 for results synthesis per study). Primary outcome measures and subgroup analyses Our primary outcomes were the overall magnitude of nocebo responses (i.e., the difference in self-reported pain/itch between a nocebo and a control trial in the evocation phase) separately for pain and itch studies employing verbal suggestions with or without classical conditioning. We thus computed 4 pooled effect sizes: verbal suggestions in pain, conditioning with verbal suggestions in pain, verbal suggestions in itch, conditioning with verbal suggestions in itch. Whenever possible, the
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