The literature on criterion referenced testing is full of discussions concerning whether classical measurement techniques are appropriate, whether variance is necessary, whether new indices of reliability are needed, and the like. What appears to be lacking, however, is a clear and simple discussion of why the problems occur. This paper suggests…

Any systematic approach to the assessment of students' ability implies the use of a model. The more explicit the model is, the more its users know about what they are doing and what the consequences are. The Rasch model is a strong model where measurement is a bonus of the model itself. It is based on four ideas: (1) separation of observable…