Measuring adjustment of siblings of children with disabilities: psychometric properties across translations, age groups and informants.

Objectives: The Negative Adjustment Scale (NAS) is used to measure adjustment to having a sibling with a disability. However, several adaptations to the scale have been made, and implementation varies across studies and countries. This study examined the psychometric properties across different versions and provides directions for future use and development of the NAS. Methods: The sample comprised 400 siblings aged 6-16 years old from Norway, the Netherlands and Belgium. Measurement invariance was assessed across age groups (8-11 vs. 12-16 years) and translations (Norwegian vs. Dutch), using a multi-group confirmatory factor analysis. The psychometric properties of the parent version were assessed among 102 parents from the Dutch sample. Results: The internal consistency of the NAS was acceptable to good in all versions. A single factor model held across age groups and translations, although higher order invariance was not supported. The child and parent report versions of the NAS did not significantly correlate. Conclusions: The NAS generally has sufficient psychometric properties. However, these vary across age groups, translations, and informants, causing the NAS to be less appropriate for comparisons between groups. Further adaptation of the NAS or developing a new scale for siblings of children with disabilities, is advised.