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.