Differential Item Functioning (DIF) is a group-related item/test characteristic targeting the issue of item/test bias. In this dissertation, a new DIF index is described that has three parts: (1) a DIF measure indicating the degree of different (item) performances between the reference and focal groups, (2) a DIF direction statement about the DIF direction and the favored groups, etc., and (3) a descriptive statement about the impact of DIF--which and how many examinees are influenced by the DIF item. For the DIF measure, one method of significance detection is described and evaluated. Then, the DIF index is applied to real data, and its results are compared to the results of other DIF methods. Furthermore, an introduction is given to the extensions of the DIF index to the test level and the polytomous item response data. Finally, some problems are evaluated and future developments are described in the discussion chapter. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]