Applying a many-facet Rasch analysis of rater effects on an oral English proficiency test, this book provides an instance of how a measurement approach offers more informative and accurate feedback on the impact of rater effects upon test scores than the classical estimates of interrater reliability commonly used in performance assessment of second language proficiency. The analysis also sheds some light on the design of more effective rater training programs with respect to quality control of rater performance in language testing. The book should be helpful for testing professionals who are involved in the rating of language tests, rater training, and test administrations, and for those who may be considering adopting Rasch analysis in performance assessment of human behavior.