The difficulty of test items that measure more than one ability

Many test items require more than one ability to obtain a correct response. This article proposes a mul-tidimensional index of item difficulty that can be used with items of this type. The proposed index describes multidimensional item difficulty as the direction in the multidimensional space in which the item provides the most information and the distance in that direction to the most informative point. The multidimensional dif-ficulty is derived for a particular item response theory model and an example of its application is given using the ACT Mathematics Usage Test. Statistics that describe the characteristics of test items are routinely used to assist in the test con-struction process. These statistics are often used to help produce equivalent forms of tests and to produce tests according to detailed specifications. Sometimes they are used to select items that will yield test forms that measure a single trait or dimension