A comparison between robust z and 0.3-Logit Difference procedures in assessing stability of linking items for the Rasch model

There are at least two procedures to assess item difficulty stability in the Rasch model: robust z procedure and ".3 Logit Difference" procedure. The robust z procedure is a variation of the z statistic that reduces dependency on outliers. The ".3 Logit Difference" procedure is based on experiences in Rasch linking for tests developed by Harcourt. Both methods were applied to archival data from two large-scale South Carolina assessment programs: HSEE 1986/1987 and PACT 2004/2005.The results of the analysis showed the ".3 Logit Difference" procedure identifies slightly more stable items (2.6%) for all items under study. In addition, approximately 93% of all items under consideration were identically classified as stable or unstable for both procedures. This very high level of agreement between the two methods indicates that either procedure can be safely used to identify stable items for use in a common-item linking design. The advantage of the robust z procedure lies in its foundation of robust statistical inference. The procedure takes into account well-accepted models for identifying outliers and permits critical values set at a specified Type I error.