Chapter 2 – Meta-analysis 43 Magnitude of nocebo responses See Figures 2 and 3 for forest and funnel plots, respectively, that display effect sizes per study and pooled effects. For pain (Figures 2A and 2B), the magnitude of nocebo responses on a standardized scale of 0-10 (with higher scores indicating larger nocebo magnitudes) across studies using classical conditioning with verbal suggestions ranged from 0.28 to 1.42, with the mean standardized response being M = 0.79 (SE = 0.24). Verbal suggestions alone induced effects on pain ranging from 0.00 to 1.27 (M = 0.70, SE = 0.30). For itch, the magnitude of nocebo responses in studies that used conditioning with verbal suggestions ranged from 0.21 to 0.47 (M = 0.35, SE = 0.24). Verbal suggestions alone induced effects on itch ranging from 0.41 to 0.75 (M = 0.58, SE = 0.26). Based on these results, on average our meta-analysis indicated medium effects of the nocebo manipulations (Hedges g between 0.26-0.71 for each of the four pooled effects), a moderate degree of heterogeneity (I2 average 41% across the four pooled effects), with the study effect sizes ranging between g = 0.00 and g = 1.34.
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