Basic issues in understanding Item Response Theory (IRT), or Latent Trait Theory, measurement models are discussed. These theories have gained popularity because of their promise to provide greater precision and control in measurement involving both achievement and attitude instruments. IRT models implement probabilistic techniques that yield…

Proposes formulas for expected true-score measures and reliability of binary items as a function of their Rasch difficulty when the trait (ability) distribution is normal or logistic. Provides an illustrative example for using the proposed formulas. (SLD)

Describes an extension to the Rasch model for fundamental measurement in which there is parameterization not only for examinee ability and item difficulty but also for judge severity. Discusses variants of this model and judging plans, and explains its use in an empirical testing situation. (SLD)