Chapter 2 – Meta-analysis 39 study inclusion, duplicates were removed, and remaining articles were considered based on title and abstract, or full text. In total, we identified 24814 articles through our searches, of which 24687 were excluded. We did not follow a strict hierarchical approach in marking exclusion criteria, but selected criteria based on what was deemed to be the major exclusion reason, for example when screening abstracts where limited information is available, therefore the following exclusion numbers provide less than precise estimates of exclusion reasons. We excluded articles for the following reasons: 8302 articles for not aiming to study nocebo or placebo effects or not using a learning paradigm to induce placebo or nocebo effects (explicit use of the terms nocebo or placebo was not an inclusion criterion), 4328 for not reporting original data or (full length, peer reviewed) experimental studies, 1229 studies for not being conducted in humans, 10440 because they were duplicates or already screened during a previous round, 101 articles for not studying (placebo/nocebo on) cutaneous sensations, 242 articles for not studying (placebo/nocebo in) healthy human participants, 20 articles because they did not report self-reported pain/itch intensity ratings, 13 for not being in English, Dutch, or German, 2 studies for not using a within- or between-subjects controlled design, 5 studies for not responding to requests for data, and 5 for excluding data from participants that were considered placebo/nocebo non-responders. A total of 127 articles were selected of which 108 included placebo conditions and 39 nocebo conditions. Of these articles, we excluded 2 observational learning studies as they were too few for a meaningful analysis. Thus, in total, 37 studies were included in this meta-analysis on nocebo effects.
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