38 mean of pain or itch ratings across the entire evocation phase was used. If only values from the first trial(s) were reported, these were used instead, and sensitivity analyses tested for differences in magnitudes between studies reporting the mean versus the first trials. We also did subgroup analyses to compare Hedge’s g between nocebo responses based on the type of learning (verbal suggestion or combination with conditioning) and type of sensory stimulation (e.g., thermal, electric) and the timing of nocebo measurement (as the mean of evocation or only the first evocation trials, by trial type). Metaregression assessed the impact of the length of learning, (quantified as the number of learning trials during induction, while we also separately examined number of trials evocation), the timing of the measurement of nocebo hyperalgesia in the evocation phase (first trials versus mean of evocation trials), the stimulus intensity (calculated as the calibrated difference in pain intensity for control vs. nocebo trials) and the Risk of Bias score on nocebo magnitudes for the included studies. Results Study selection Figure 1 shows the flow of the study selection process including the reasons for exclusion at each stage. A total of 17546 nocebo and placebo papers were initially identified through the database searches. We searched for more eligible studies through reviewing the reference lists as well as web of science for each included study, as well as conducting repeat database searches in June 2020 and July 2021. At each stage of
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