190 Chapter 7 4.1 Validity For testing convergent validity, we choose the widely-used disease specific PROMs within hemophilia pediatric care and research (Haemo-QoL and PedHAL) [20]. These PROMs aim to measure the effect of hemophilia on daily life, and specifically ask if children experience symptoms like pain, sadness, or problems with friends due to their hemophilia (eg, I was angry because of my hemophilia). This is different from the PROMIS pediatric instruments that measure a generic domain of health and assume that symptoms can occur due to multifaceted reasons (eg, I was angry) [19]. Due to these different approaches, strong correlations were not expected, and it was hard to accurately assess convergent validity. For example, the PROMIS Peer Relationships item bank was expected to correlate minimally with the Haemo-QoL Other People scale as they assess different domains of social health. The Other People scale of Haemo-QoL relates more to the ability to participate in social roles due to hemophilia, whereas the Peer Relationships item banks relates to the overall quantity and quality of relationships with peers. Similarly, the Haemo-QoL Feeling scale does not cover the same unidimensional domains as measured by the PROMIS item banks. Nonetheless, most convergent validity hypotheses were met in both age groups, except for PROMIS Peer Relationships and PROMIS Depressive Symptoms item banks and the Global Health scale for 13-17 year olds. Previous studies have shown that the subjective questioning of the Global Health scale (“How would you rate your own health?”) may be influenced by social norms, which could be a possible explanation for a low correlation with the more objective questioning of the Haemo-QoL (total score of all subdomains), which relates much more to reported symptoms [53]. In addition, the correlation between the PROMIS pediatric measures and the legacy instruments could be negatively affected by the high floor and ceiling effects and the differences in recall period [33]. The PROMIS instruments use a recall period of 7 days, while the legacy instruments apply recall periods of 4 weeks/month [54]. A limitation of this study is that we were unable to directly compare the reliability of the PROMIS instruments and legacy instruments, due to the use of different measurement theories (IRT vs CTT). Results showed that both the PROMIS pediatric measures as well as the legacy instruments measure reliably. However, higher floor and ceiling effects were found for the legacy instruments than for the PROMIS item banks (except for the PROMIS Mobility item bank) negatively affecting content validity and reliability. This in accordance with previous studies on the PedHAL and Haemo-QoL instruments, where floor and ceiling effects were also found [42,43].
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