Item response theory (IRT) is a mental measurement theory based on the postulate that an individual's response to a test item is a probabilistic function of characteristics of the person and characteristics of the item. The person characteristics are the individual's levels of the traits being measured, and the item characteristics are features such as difficulty and discriminating power. Item response theory has several advantages over classic test theory and has the potential to solve several difficult measurement problems. The foundations of item response theory were developed in the early 20th century; however, it was Frederic Lord, beginning in the 1950s, who organized and developed the theory into a framework that could be applied to practical testing problems. Advances in computing were necessary to make ...