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In a typical ability test, groups of items are administered, and the number of items an examinee answers correctly is used to estimate his/her ability. The more items an individual answers correctly, the greater his/her ability is assumed to be. However, because everyone responds to every item, most examinees are administered items that are either too easy or too difficult. Adding these items to the test is similar to adding constants to a score; they provide relatively little information about the examinee’s ability level.

In Computer Adaptive Testing (CAT), the estimated ability level of the examinee is used to predict the probability of getting an item correct. With no knowledge about an examinee in the beginning, it is assumed he/she is of average ability. CAT begins by administering an item of average difficulty. An examinee who correctly answers the first item is then given a more difficult item; if that item is answered correctly, the computer administers an even more difficult item. Conversely, an examinee who gets the first item wrong is administered an easier question. In short, the computer interactively adjusts the difficulty of the items administered based on the success or failure of the test taker.