Performance of the Generalized S‐X² Item Fit Index for Polytomous IRT Models

Orlando and Thissen's S‐X 2 item fit index has performed better than traditional item fit statistics such as Yen's Q 1 and McKinley and Mill's G 2 for dichotomous item response theory (IRT) models. This study extends the utility of S‐X 2 to polytomous IRT models, including the generalized partial credit model, partial credit model, and rating scale model. The performance of the generalized S‐X 2 in assessing item model fit was studied in terms of empirical Type I error rates and power and compared to G 2 . The results suggest that the generalized S‐X 2 is promising for polytomous items in educational and psychological testing programs.