Information Functions of the Generalized Partial Credit Model

The concept of information functions developed for dichotomous item response models is adapted for the partial credit model. The information func tion 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 informa tion function then is used to investigate the effect of collapsing and recoding categories of polytomously-scored items of the National Assess ment 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.