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Chapter 6146United States), and Cincinnati Children’s Hospital Medical Center (Cincinnati, United States). Additionally, participants were recruited via the Dutch (STSN), Belgian (beTSC), and United States (TSC Alliance) patient organizations. MeasuresIn addition to the TSC-PROM, the SF-36,24 including a proxy-report,42 and scales assessing emotional and behavioral problems from the Achenbach System of Empirically Based Assessment (ASEBA),43,44 i.e., the Adult Self Report (ASR), the Child Behavior Checklist (CBCL)/1.5-5 and CBCL/6-18 were used for assessing construct validity. Individuals who were mentally competent to fill out the questionnaires themselves received the ASR. For individuals with TSC who could not complete the questionnaires, caregivers or representatives indicated whether the developmental age was below or above the age of six years old, guiding the distribution of either the CBCL/1.5-5 or CBCL6-18 version. Information on measurement properties of these comparator instruments is provided (see Additional file 1).24,27,38,42–62Item reductionItem reduction was performed by selection based on frequency (at least 85% with response option ‘Not at all’), factor loadings, monotonicity, or local independence unless there was a clinical reason to include the item based on expert opinion. Additionally, items of the proxy version were reduced when frequency of the response options ‘Don’t know’ and ‘Not applicable’ was at least 30%. Statistical analyses Statistical analyses were performed using IBM SPSS Statistics 24 and R. Descriptive statistics were used to characterize demographics, clinical variables, and score distributions of the TSC-PROM. Domain scores were calculated as a percentage of the sum of the items within a domain. For the social support domain, the sum was divided by the number of items filled in other than ‘unknown’ or ‘not applicable’ times the number of response options to account for the ‘unknown’ and ‘not applicable’ response options. The TSC-PROM total score was the average of the domain percentages, excluding the social support domain. The social support domain was Annelieke Muller sHL.indd 146 14-11-2023 09:07