The Rasch measurement model improves on traditional test construction by creating tests in which the person's ability is independent of the sample of items used and the norm group used to calibrate the test. This paper reviews the Rasch model by describing properties of the item characteristic curve (ICC) and discussing the utility of having person ability and item difficulty on a common scale. Calibrating item difficulty and person ability to a common scale allows test developers and instructors to create better instruments in terms of optimizing the number of items, eliminating items of the same difficulty, and more closely matching the level of difficulty of the items with the abilities of the examinees. (Contains 1 table, 2 figures, and 12 references.) (SLD)