CHAPTER 7 180 Discussion The aim of this study was to evaluate the reliability and validity of the PRPP-Assessment by parent-provided videos (n=65) in 13 children with a mitochondrial disorder, as rated by 11 therapists. Intra-rater reliability was sufficient to good. The internal consistency was high, which shows that the items of stage 2 consistently reflect the information processing model in children with a mitochondrial disorder. Also, the ICCAgreement on the Mastery score (stage 1) showed acceptable inter-rater reliability. As anticipated, the ICCs to determine inter-rater reliability were low due to the lack of variation between cases on stage 2. However, the standard deviation of the variance component of raters was lower than the variance of the children which indicates acceptable inter-rater reliability. Although convergent validity based on comparing COPM and PRPP could not be established, good structural and known group validity were demonstrated, resulting in four out of six accepted hypothesis on construct validity. In addition, Rasch analysis demonstrated sound goodness-offit. Overall, results support the reliability and validity of the PRPP-Assessment in the heterogenic group of children with mitochondrial disorder. Comparing reliability results to existing literature shows similarities and differences. For example, the high internal consistency found in the current study was also observed in other target groups: Nott et al.[21] found an internal consistency of 0.88 in adults with brain injury; Van Keulen-Rouweler et al.[28] found an internal consistency of 0.71 in persons with Parkinson’s disease. For rater reliability, the findings seem in line with previous studies in other target groups, but comparison is hampered due to different methods in statistics and in using free versus fixed procedural task-analysis. For instance, Van Keulen-Rouweler et al.[28] used ICC’s, and found a moderate to good intra-rater reliability (0.6-0.83), although with a wide range. This wide range is also observed in the current study, even though very few datapoints outside the 95% Limits of Agreement were found; overall 32 out of 484. In addition, Nott et al.[21] calculated intra-rater reliability for stage 2 Sumscores in adults with brain injury by using Limits of Agreement, as also used in the current study. They found a slightly larger mean percentage difference of -4.52. Although all datapoints of the study of Nott et al.[21] were within the 95% Limits of Agreement, the differences of the current study are more concentrated around the zero. Focusing on inter-rater reliability, Van Keulen-Rouweler et al.[28] found a relatively low ICC for stage 1 mastery score in persons with Parkinson’s disease (0.36) without the use of a pre-set task analyses. Consequently, they recommended to use a pre-set procedural task analysis in line with clinical practice and also adopted in this study. Using a pre-set task analysis in the current study led to a more acceptable ICC for stage 1 mastery score (0.65) in line with the study of Steultjens et al.[31], who
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