Chapter 4 – Experimental learning 113 Materials and Methods Participants One hundred and forty participants were enrolled in this study. The required sample size for the primary analysis was calculated based on a previous similar nocebo study 15. The analysis was conducted in G*power 3.1 22 for a mixed model ANOVA. The effect size was f = 0.26, alpha error probability was set at α = 0.05, desired power was set at 0.95, and the correlation for repeated measures was set at 0.05 (because of the subjectivity and high variability expected in pain ratings). According to the total sample size indicated, we planned for 140 participants to be enrolled, of which a total of 122 participants were included in the study. This sample size for the primary hypothesis is similar to previous studies examining subtle between-groups differences such as conditioning with partial reinforcement or anxiety correlates 15,23. The main groups were split in half for the purposes of some of the secondary analyses in this two-by-two design, resulting in subgroups of 24-25 participants, a sample size that has been used in previous nocebo studies that yielded significant results with good effect sizes 21,24,25. Participants were required to be between 18 and 35 years old, have a good understanding of the Dutch language as well as (corrected to) normal vision and hearing. Exclusion criteria were serious medical or psychiatric conditions, pregnancy, painful health conditions experienced in the past 6 months, and pain or use of analgesic medication at the time of testing. Participants who were determined to have too high of a threshold for pain upon their visit to the department (i.e., when thermode maximum temperatures were not sufficient to induce at least moderate pain) were also excluded from the study. All participants were asked to refrain from alcohol and caffeine consumption, as well as the use of drugs and analgesic medication, in the 12 hours before the testing appointment. Participants were recruited via flyers, social media
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