Chapter 2 – Meta-analysis 51 measured nocebo responses as the mean of all evocation (testing) trials, 6 measured the magnitude of responses in the first pair of evocation trials, and 2 studies specified different timing such as pre-post measures. Studies in which first evocation trials were used yielded a large pooled effect on the magnitude of nocebo responses (k = 6, g = 0.82, 95% CI 0.57 – 1.07) compared to medium pooled effects of measuring the effect as the mean of all evocation trials (k = 13, g = 0.66, 95% CI 0.54 – 0.79) and non-specified (k = 2, g = 0.67, 95% CI 0.23 – 1.11). Magnitude of nocebo hyperalgesia based on the number of learning trials Studies that employed classical conditioning used varying numbers of learning and evocation trials. For pain only, there were sufficient studies to examine the effects of different lengths of conditioning and different lengths of evocation (i.e., the length of extinction) on nocebo magnitudes. The shortest pain learning paradigm used 6 nocebo and 6 control trials, while the longest paradigms used up to 30 nocebo and 30 control trials. Evocation phases ranged from 3 nocebo and 3 control trials to 30 nocebo and 30 control trials. A meta-regression of different lengths of conditioning showed no association with the magnitude of nocebo responses (Q = 0.81, p = 0.37). Similarly, there was no association between the length of evocation and nocebo magnitudes (Q = 0.19, p = 0.67). Magnitude of nocebo hyperalgesia based on the pain stimulus intensity For pain studies that employed classical conditioning with verbal suggestions we had a sufficient sample to examine any relationship between differences in intensity of pain stimulations in the learning
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