Efforts to measure intelligence have long been a part of psychology, and despite controversy over the meaning and scope of intelligence, an IQ score can provide meaningful data about a child’s cognitive abilities if put within a conceptual framework that does not overstate its meaning or implications for the child. Intelligence tests are the most studied and, consequently, the most reliable, valid, and useful tests available for measuring specific cognitive abilities. Within a particular IQ test, children tend to perform the same on items designed to assess the same ability, which suggests internal consistency. The tests are reliable because children generally receive the same score when they retake the same test years later, although the reliability of the test usually increases with the age of the child. Test validity is based on numerous studies that have found high correlations between children’s IQ scores and their performance in school, achievement tests, and tests of specific intellectual functioning (eg, measures of language, visual motor processing).
An IQ score reflects a child’s performance on an intelligence test relative to that of children of the same age. In short, a child’s IQ score tells the extent to which his or her performance on the test departs from average. The IQ score represents a construct of “intelligence” that includes a combination of verbal and nonverbal processing skills, such as vocabulary, information about the world, reasoning, short-term memory, and speed of information processing; these skills, together, are represented by the IQ score. Nearly all comprehensive psychological evaluations include some measure of intelligence. For example, for a child who is being tested to confirm a diagnosis of attention-deficit/hyperactivity disorder (ADHD), an intelligence test can confirm that the child’s academic difficulties do not indicate a specific cognitive weakness or mild mental retardation.
Most intelligence tests assess a range of verbal, visual-spatial, and problem-solving skills. Because they target multiple cognitive skills, IQ tests are composed of subtests that measure specific areas of functioning. Scores on these subtests are combined to yield measures of verbal and nonverbal problem-solving abilities, as well as a full-scale IQ score. IQ scores are assumed to be normally distributed in the population, with most scores falling in the middle of the distribution and fewer scores falling at the upper and lower extremes (Figure⇓). The average IQ score on most IQ tests is 100, with a standard deviation of 15. Most IQ scores (about 68%) fall within 1 standard deviation on either side of the mean (eg, between 85 and 115), and almost all scores (99% of population) fall within 3 standard deviations above or below the mean.