A graphical-decision rule for studying an item's behavior and admitting items onto a test form is proposed. The tool--the isthmus of acceptance--ameliorates some parameter stability problems common to binary item response theory models by using a functional representation of item performance, and it is tuned to match current standards. (TJH)

It is important to estimate the number of examinees who reached a test item, because item difficulty is defined by the number who answered correctly divided by the number who reached the item. A new method is presented and compared to the previously used definition of three categories of response to an item: (1) answered; (2) omitted--a…