Cross-Cultural Adaptation and Psychometric Evaluation of Greek Nutrition for Sport Knowledge Questionnaire (NSKQ-Gr): Evidence from Classical Test Theory and Rasch Modeling

Background/Objectives: Valid and culturally adapted instruments are essential for accurately assessing sports nutrition knowledge among coaches, given their influential role in shaping athletes’ dietary behaviors and performance outcomes. This study aimed to translate, culturally adapt, and evaluate the psychometric properties of the Nutrition for Sport Knowledge Questionnaire (NSKQ) for Greek certified coaches, while also examining knowledge levels and associated demographic factors. Methods: A cross-sectional study was conducted among 514 certified coaches representing various sports disciplines. The translation followed a standardized forward–backward procedure with expert panel review to ensure cultural relevance. The psychometric properties of the NSKQ-Gr were evaluated using complementary Classical Test Theory (CTT) and Rasch approaches. Item difficulty, item discrimination, Kuder–Richardson Formula 20 (KR-20), McDonald’s omega, and confirmatory factor analyses (CFAs) using the WLSMV estimator for dichotomous items were performed. Test–retest reliability was examined via Pearson’s correlation coefficient and intraclass correlation coefficient (ICC). Statistical analyses explored associations between nutrition knowledge and demographic and professional characteristics. Results: Item discrimination varied across domains; the strongest performance was observed for the Alcohol domain and the weakest for Weight Management and Sports Nutrition. KR-20 and McDonald’s omega indicated moderate internal consistency for most domains, apart from Weight Management and Sports Nutrition, the reliability of which was lower. The temporal stability was excellent (ICC = 0.998). CFA findings were heterogeneous across domains, whereas Rasch analyses showed generally acceptable item-level fit. Most coaches demonstrated average (56.8%) or poor (32.3%) sports nutrition knowledge. Good or excellent knowledge was demonstrated by only 10.9% of the participants. Knowledge scores differed according to age, certification pathway, previous athletic experience, and several nutrition-related coaching practices. Conclusions: The NSKQ-Gr provides a useful multidomain instrument for assessing sports nutrition knowledge among Greek coaches. The present study extends previous validation work by providing complementary evidence from CTT, CFA, and Rasch modeling, thereby offering a more comprehensive psychometric evaluation of the NSKQ-Gr. The findings indicate that the NSKQ-Gr is a multidomain knowledge assessment instrument suitable for Greek coaches. Future validation studies are encouraged to further refine its psychometric properties.