34 example visceral pain studies. For the purposes of in- and exclusion, studies were considered to have induced a placebo or nocebo effect if a learning paradigm was used to induce positive or negative outcome expectations about an inert treatment. We considered as nocebo learning paradigms only those that aimed to induce negative expectations regarding an intervention, such as sham electrical stimulation or an inert cream. This meant that most conditioning without verbal suggestion studies were excluded from this review, as they did not include treatment associations, and were considered to be pain-conditioning, not noceboconditioning studies (albeit explicit mention of the terms nocebo and placebo was not a specific inclusion criterion). Additionally, we only included studies that had a control group or a control condition withinsubjects, so that nocebo effects could be calculated as the difference between nocebo and control/no treatment on self-reported scores. We excluded studies that excluded or did not report data from nocebo nonresponders. Post-hoc, we excluded observational learning studies as they were too few for a meaningful analysis. Studies that did not fulfill one or more of the criteria mentioned above were excluded from the metaanalysis (see Figure 1 for a flow diagram). Study selection Eligibility assessment for the inclusion of studies was performed independently by two authors in each of the following steps. Titles and abstracts of articles retrieved using the search strategy were screened by two authors independently (M.M.E.v.S. and J.S.B.). The full text of articles to be included and articles about which doubts existed were then retrieved and assessed for eligibility by two authors independently (M.A.T. and J.S.B.). The reference lists of all included articles were also screened for study inclusion by two authors (M.A.T. and J.S.B.) and included articles were also entered in Web of Science to identify articles
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