Basal Level and Ceiling Level

The basal level (also called the floor level) and the ceiling level are components of the termination criteria used in Binet-type individually administered adaptive tests that are used to measure IQ in educational and other settings. These tests are administered by psychologists or trained examiners and operate from a question/item bank that is organized into mental age levels. Mental age is defined for each test item as the average chronological age at which approximately 50% of the standardization examinees correctly answered the item.

The adaptive test is begun by the examiner by selecting a mental age level based on the examiner’s knowledge of the child being tested. This information can simply be the child’s chronological age or can be based on other information such as a teacher’s statement about the child’s probable IQ (e.g., she is an “above average” student or he is “below average” or “average”). Having selected a mental age level to begin the test, the examiner administers each item at that level and scores the result as correct or incorrect.

When all items at this entry level have been administered, the score for that level is tallied. If the child has answered all items at that level correctly, the examiner has identified the basal level for that test and has identified the upper limit of the portion of the item bank that is too easy for the child. If the basal level has not been identified, the examiner then has the option of moving to the next higher or lower mental age level and administering the items at that level. Again, the proportion correct is tallied and a decision is made based on that information. If the basal level had not yet been identified, the test can continue with items at lower mental age levels until the child correctly answers all items at a given mental age level.

Alternatively, the examiner can first seek the ceiling level for that child—the mental age level at which all items are answered incorrectly, thus identifying the lower limit of the portion of the item bank that is too difficult for the child. Once both the basal level and the ceiling level have been identified, the test is terminated and the portion of the item bank that provides effective measurement for the child has been identified.

The IQ score that is computed from this procedure is based on a weighted function of the mental ages of the items answered correctly between the basal and ceiling levels. Because the items administered in the adaptive test are selected from the item bank based on the child’s performance as the test is administered, the scores resulting from this type of test have greater measurement precision than those from tests in which all examinees receive the same set of items, many of which might not be appropriate for a given examinee.