Effects of variations in item step values on item and test information in the partial credit model

Abstract

Simulated data were used to investigate systematically the impact of various orderings of step difficulties on the distribution of item information for the partial credit model. It was found that the distribution of information for an item was a function of (1) the range of the step difficulty values, (2) the number of step difficulties that were out of sequential order, and (3) the distance between the step values that were out of order. Also, by using relative efficiency comparisons, the relationship between the step estimates and the distribution of item information was used to demonstrate the effects of various test revisions (through the addition and/or deletion of items with specific step characteristics) on the resulting test’s precision of measurement. The usefulness of item and test information functions for specific measurement applications of the partial credit model is also discussed.