Common Instruments That Measure Intelligence
A discussion of all possible IQ tests is beyond the scope of this book, so we will focus on the three most commonly used measures. The selection of which measure to use will be based on the psychologist's preference and the circumstances surrounding the assessment. If a child has a language deficit, for example, the psychologist may want to use the Differential Ability Scale, as it contains a special nonverbal scale. The Universal Nonverbal Intelligence Test may also be used because this instrument requires only motor responses (for example, point to an answer or construct a design).
Depending on the child's age, one of three scales can be administered: Wechsler Preschool and Primary Scale of Intelligence—Third Edition (WPPSI-III: ages 2 years, 6 months to 7 years, 3 months); Wechsler Intelligence Scale for Children—Fourth Edition (WISC-IV: ages 6 years to 16 years, 11 months); and Wechsler Adult Intelligence Scale—Fourth Edition (WAIS-IV: ages 16 to 90 years). Because the WISC-IV is the alternative that would apply to the majority of school-age children, we will focus on this particular test.
The WISC-IV (2003) is the fourth revision of the Wechsler scales for this age range. The test provides an overall Full Scale IQ, as well as four index scores: Verbal Comprehension Index, Perceptual Reasoning Index, Working Memory Index, and Processing Speed Index. Full Scale IQ and index scores are reported as standard scores (mean of 100, standard deviation of 15). Information about the different subtests, examples of items, and descriptions of what the tests measure are presented in Table 7.4.
The Verbal Comprehension Index is a measure of a child's verbal reasoning, verbal concept formation, word knowledge, and practical or social judgment.
The Perceptual Reasoning Index involves tasks of perceptual reasoning that require minimal or no verbal response and measure nonverbal concept formation and visual problem-solving ability. Visual motor performance (speed) can earn bonus points in constructing block designs.
The Working Memory Index is a measure of a child's ability to mentally hold information in short-term memory while manipulating (performing an operation on) the information in some way, such as when doing mental math problems.
The Processing Speed Index measures a child's speed of copying or scanning visual information, or both.
The following case study of Jason will help to illustrate how intelligence test scores can assist in better understanding learning difficulties.
Jason's Full Scale IQ on the WISC-IV was 87 (Range 82–92), which is at the upper limits of the Low Average range. Based on Jason's overall IQ score, his learning potential would not predict serious academic difficulties. However, the index scores reveal a pattern of strengths and weaknesses that can help us in understanding why he is having problems. Jason's index scores ranged from a high of 102 (Average) for Perceptual Reasoning to a low of 78 (Borderline) for Processing Speed; a discrepancy of this magnitude (24 points) is highly significant, occurring in less than 1 in 20 students.
Although Jason demonstrates an obvious strength in Visual/Perceptual Reasoning, he takes a significant amount of time to copy and scan information (Processing Speed Index).
The individual subtests that make up the four index scores (see the lower portion of Table 7.5) are presented as scaled scores with a mean (average) score of 10 and standard deviation (unit of comparison) of 3. Therefore, average scale scores range from 8 to 12. Any scores below 8 or above 12 deserve extra consideration. Jason has significant problems seeing how verbal concepts and ideas are related (Similarities) and, to a lesser extent, how several visual pictures might share a common theme (Picture Concepts). Based on these scores, we would anticipate that Jason would have problems using examples from life experiences to help make concepts more meaningful to him, unless directed to do so.
Another area of weakness would be the actual time it takes Jason to complete tasks (Processing Speed) compared with other children in the classroom. His strengths in visual/perceptual reasoning suggest that the use of visual symbols is one method that could assist Jason in better understanding the relationships that exist between concepts. Based on the results of Jason's performance on the IQ test, the school psychologist can now assess academic performance to determine how closely these scores match what would be predicted given his Full Scale IQ.