Psychometric Evaluation of the Naglieri Nonverbal Ability Test (NNAT-I) in Turkish Middle School Students Aged 10-14 Years.

The Naglieri Nonverbal Ability Test-Individual Form (NNAT-I) is widely used to assess nonverbal reasoning while minimizing language demands, yet evidence regarding its psychometric properties in Turkish middle school students is limited. This study examined the reliability, convergent validity, internal structure, item functioning, and differential item functioning (DIF) of the NNAT-I in 1130 students aged 10-14 years attending schools across socioeconomic development strata in Aydın, Türkiye. Internal consistency, test-retest and alternate-form reliability, convergent validity, confirmatory factor analysis (CFA), Rasch analyses, multiple-group Rasch DIF analyses, and linear mixed-effects modeling were conducted. The NNAT-I demonstrated high internal consistency (Cronbach's α = 0.93; McDonald's ω = 0.94), strong test-retest and alternate-form reliability, and strong correlations with the Test of Nonverbal Intelligence-Third Edition (TONI-3) and Raven's Standard Progressive Matrices (RSPM). CFA provided qualified rather than unequivocal evidence for a predominantly unidimensional structure, with model fit varying according to item response distributions and the treatment of structurally nonadministered responses. Rasch analyses of the calibrated items supported an ordered hierarchy of item difficulty, with all items showing Infit values within the prespecified descriptive reference range. After adjustment for multiple comparisons, DIF analyses identified significant gender-related DIF for only one item, while no items showed significant age-related DIF. Linear mixed-effects modeling showed that grade level and gender were significantly associated with NNAT-I scores, whereas chronological age was not independently associated with performance. These findings provide evidence regarding the reliability, convergent validity, internal structure, and item functioning of the NNAT-I in this Turkish middle-school sample, while also indicating that conclusions regarding unidimensionality should remain qualified. The findings underscore the importance of local psychometric evaluation before applying nonverbal measures in new educational and cultural contexts.