Use of Robust z in Detecting Unstable Items in Item Response Theory Models.

The first part of this paper describes the use of the robust zR statistic to link test forms using the Rasch (or one-parameter logistic) model. The procedure is then extended to the two-parameter and three-parameter logistic and two-parameter partial credit (2PPC) models. A real set of data was used to illustrate the extension. The linking results illustrate the efficacy of the robust zR vis-a-vis some of the most commonly used processes such as the Stocking and Lord (1983) linking process.