IRT is a set of mathematical procedures for estimating the item parameters as well as the placement of individuals along some latent trait. A key concept in IRT is that item parameters define an item characteristic curve (ICC), a function that relates the latent trait score to probability of a positive score on that indicator. In its simplest form, the Rasch model, the ICC function is estimated by one parameter for each indicator: a measure of where on the latent trait the item provides maximal discrimination (sometimes referred to as threshold, severity, or “difficulty” on ability measures).
