Unique properties of Rasch model item information functions

The Rasch family of models displays several well-documented properties that distinguish them from the general item response theory (IRT) family of measurement models. This paper describes an additional unique property of Rasch models, referred to as the property of item information constancy. This property asserts that the area under the information function for Rasch models is always equal to the number of response categories minus one, regardless of the values of the item location parameters. The implication of the property of item information constancy is that, for a given number of response categories, all items following a Rasch model contribute equally to the height of the test information function across the entire latent continuum.