Information functions of the generalized partial credit model

Abstract

The concept of information functions developed for dichotomous item response models is adapted for the partial credit model. The information function is explained in terms of the model parameters and scoring functions. The relationship between the item information function and the item response function also is discussed. The information function then is used to investigate the effect of collapsing and recoding categories of polytomously-scored items of the National Assessment of Educational Progress (NAEP). The NAEP writing items were calibrated and the item and test information is used to discuss desirable properties of polytomous items. Index terms: information function, item response model, National Assessment of Educational Progress (NAEP), partial credit model, polytomous item response model.