for Fixed-Anchor Item Calibration

The nonequivalent groups anchor test (NEAT) design is often used to scale item parameters from two different test forms. A subset of items, called the anchor items or common items, are administered as part of both test forms. These items are used to adjust the item calibrations for any differences in the ability distributions of the groups taking different forms. One method for scaling the item parameters is the fixed-anchor item method, in which the parameters for the anchor items are held constant at the values calibrated in the base form. With this method, the parameters for the new form should be on the metric of the old form, with no need for further scaling. The fixed-anchor procedure may be more complicated when the base-form and new-form ability distributions show large differences, as they would for vertical scaling. Several studies have demonstrated that in BILOG (Zimowski, Muraki, Mislevey, & Bock, 2003), if one fixes the anchor item parameters and specifies NOADJUST on the CALIB line, and if the new-form ability distribution is not close to N(0,1) relative to the metric of the anchor items, the scaling for the new-form nonanchor items and the new-form ability distribution will not be accurate (Kang & Peterson, 2009; Kim, 2006)