Construct validity of the Nutrition and Activity Knowledge Scale in a French sample of adolescents with mild to moderate intellectual disability.

The purpose of this study was to test the reliability (i.e. internal consistency and test-retest reliability) and construct validity (i.e. content validity, factor validity, measurement invariance, and latent mean invariance) of the Nutrition and Activity Knowledge Scale (NAKS) in a sample of French adolescents with mild to moderate Intellectual Disability (ID). A total sample of 260 adolescents (144 boys and 116 girls), aged between 12 and 18 years old, with mild to moderate ID was involved in two studies. In the first study, analysis of items' content reveals that many words from the original version were not understood or induced confusion. These items were reworded and simplified while retaining their original meaning. In the second study, results provided support for: (i) the factor validity and reliability of a 15-item French version of the NAKS; (ii) the measurement invariance of the resulting NAKS across genders and ID levels; (iii) the partial measurement invariance of the resulting NAKS across age groups and type of school placement. In addition, the latent means of the 15-item French version of the NAKS proved to be invariant across gender, age categories, and ID levels, but to vary across type of school placement (with adolescents schooled in self-contained classes from regular schools presenting higher levels of NAK than adolescents placed in specialized establishments). The present results thus provide preliminary evidence regarding the construct validity of a 15-item French version of the NAKS in a sample of adolescents with ID.