Detecting measurement disturbance effects: the graphical display of item characteristics

Traditional identification of misfitting items in Rasch measurement models have interpreted the Infit and Outfit z standardized statistic. A more recent approach made possible by Winsteps is to specify "group = 0" in the control file and subsequently view the item characteristic curve for each item against the true probability curve. The graphical display reveals whether an item follows the true probability curve or deviates substantially, thus indicating measurement disturbance. Probability of item response and logit ability are easily copied into data vectors in R software then graphed. An example control file, output item data, and subsequent preparation of an overlay graph for misfit items are presented using Winsteps and R software. For comparison purposes the data are also analyzed using a multi-dimensional (MD) mapping procedure.