Seeing the forest for the trees: prevalence of low scores on the Wechsler Intelligence Scale for Children, fourth edition (WISC-IV).

Low scores across a battery of tests are common in healthy people and vary by demographic characteristics. The purpose of the present article was to present the base rates of low scores for the Wechsler Intelligence Scale for Children, fourth edition (WISC-IV; D. Wechsler, 2003). Participants included 2,200 children and adolescents between 6 and 16 years of age from the WISC-IV U.S. standardization sample. Measures considered in the base rates analyses included the 10 core subtests and the 4 index scores. Analyses were conducted for the entire standardization sample as well as stratified by different classifications of intelligence and different years of parental education. In the total sample, it is uncommon to have 6 or more subtest scores or 2 or more Index scores <or= 9th percentile. The prevalence of low scores typically increased with lesser intelligence and fewer years of parental education (e.g., children with below-average intelligence were 75 times more likely than children with above-intelligence to have at least one impaired subtest score). Consistent with existing studies of the base rates of low scores, some low scores on the WISC-IV were common in children and adolescents, and the frequency was related to a child's level of intelligence and parental education.