AN EMPIRICAL-EXAMINATION OF THE IRT INFORMATION OF POLYTOMOUSLY SCORED READING ITEMS UNDER THE GENERALIZED PARTIAL CREDIT MODEL

Using Muraki's (1992) generalized partial credit IRT model, polytomous items (responses to which can be scored as ordered categories) from the 1991 field test of the NAEP Reading Assessment were calibrated simultaneously with multiple-choice and short open-ended items. Expected information of each type of item was computed. On average, four-category polytomous items yielded 2.1 to 3.1 times as much IRT information as dichotomous items. These results provide limited support for the ad hoc rule of weighting k-category polytomous items the same as k - 1 dichotomous items for computing total scores. Polytomous items provided the most information about examinees of moderately high proficiency; the information function peaked at 1.0 to 1.5, and the population distribution mean was 0. When scored dichotomously, information in polytomous items sharply decreased, but they still provided more expected information than did the other response formats. For reference, a derivation of the information function for the generalized partial credit model is included.