Significantly less time is needed to administer CATs than a fixed-item test since fewer items are needed to achieve acceptable accuracy. Shorter testing times also reduce fatigue, which can be a significant factor in students' test results. By including more relatively easy and more relatively difficult items in the item pool, CATs can accommodate the abilities of both bright and slow students. CATs should not be used for some subjects and skills. Most CATs are based on an item-response theory model, which assumes that all the information needed in selecting items can be summarized in one to three parameters that describe the item's difficulty for students who have different abilities.
Many tests cover a number of different skills or topics, however.
Specifications for traditional tests seek to ensure an even range across skills or topics. Most common CAT strategies do not accommodate such additional considerations. Hardware limitations further restrict the types of items that can be administered by computer. Items involving detailed art work and graphs or extensive reading passages, for example, are hard to present using the types of computers found in most schools. Another limitation of CATs stems from the need for careful item calibration.