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Item response theory models arose from the inherent limitations of classical test theory methods of test analysis. A brief description of those limitations and the corresponding enhancements provided by item response models is provided. Further, an examination of the popular Rasch one-parameter latent trait model is undertaken. Specific explanation of the step-by-step calculations in the one-parameter model is accomplished using a commonly available spreadsheet. This paper is designed to be used as a teaching heuristic to assist students in understanding both the mechanics and the rationale behind the item response theory model measurement. (Contains nine tables, four figures, and nine references.) (Author/SLD) * Reproductions supplied by EDRS are the best that can be made * * from the original document. * C