Item response curves (item characteristic curves) are used to describe the relationship between the ability, defined on an ability scale, and each item. An item response curve plots the probability of correctly answering an item against different levels of ability. An item with perfect discrimination has a 0% probability of correct answers for respondents with ability below a threshold and a 100% probability of a correct response for subjects with ability above the threshold (Characteristic Curve for an Item with Perfect Discrimination).
Characteristic Curve for an Item with Perfect Discrimination
Typical Item Response Curve

Help created on 9/19/2017