In psychometrics, item response theory (IRT) also known as latent trait theory, or modern test theory, is a paradigm for the design, analysis, and scoring of tests, questionnaires, and similar instruments measuring abilities, attitudes, or other variables. IRT is an array of mathematical models that describe how an examinee responds to an item probabilistically as a function of the trait level of the examinee and the characteristics of the item (e.g., difficulty and discriminating power). In IRT, the examinee’s score is defined on the latent (unobservable) trait rather than on the test, as is the case in classical test theory. Therefore, the IRT trait score is considered independent of the particular set of items, whereas the true score in classical test theory is bound to the particular set of items.