Usability evaluation methods have a long history of research. Latest contributions significantly raised the validity of method evaluation studies. But there is still a measurement model lacking that incorporates the relevant factors for inspection performance and accounts for the probabilistic nature of the process. This paper transfers a modern probabilistic approach from psychometric research, known as the Item Response Theory, to the domain of measuring usability evaluation processes. The basic concepts, assumptions and several advanced procedures are introduced and related to the domain of usability inspection. The practical use of the approach is exemplified in three scenarios from research and practice. These are also made available as simulation programs.