The basic Rasch measurement model is both strict and simple. It prescribes the two attributes that matter when the interaction between a person and an item is modeled: the ability (agreeability) of the person and the difficulty (endorsability) of the item. This chapter includes many-facets Rasch model (MFRM) exemplar, in which Engelhard examined rater severity and other aspects of the writing ability assessment in the high-stakes eighth grade writing test administered annually to all eighth-grade students in the state of Georgia. The many-facets model allows all the relevant facets of a measurement situation to be modeled concurrently in order that they can be examined independently. The lack of invariance termed 'differential item functioning' in the less complex models of the Rasch family can be generalized to the concept of 'differential facet functioning' (DFF) in the MFRM to detect whether the invariance expected under the model's requirements actually are instantiated empirically or whether some sort of bias exists.