IQ and mental deficiency. A person is diagnosed with a mental deficiency if, before the ag...

IQ and mental deficiency. A person is diagnosed with a mental deficiency if, before the age of 18, his or her score on a standard IQ test is no higher than 70 (two standard deviations below the mean of 100). Researchers at Cornell and West Virginia Universities examined the impact of rising IQ scores on diagnoses of mental deficiency (MD) ( American Psychologist , October, 2003). IQ data were collected from different school districts across the United States, and the students were tested with either the Wechsler Intelligence Scale for Children—Revised (WISC-R) or the Wechsler Intelligence Scale for Children—Third Revision (WISC-III) IQ tests. The researchers focused on those students with IQs just above the mental deficiency cutoff (between 70 and 85), based on the original IQ test. These “borderline” MD students were then retested one year later with one of the IQ tests. The accompanying table gives the number of students diagnosed with mental deficiency on the basis of the retest. These data are saved in the MDIQ file. Conduct a chisquare test for independence to determine whether the proportion of students diagnosed with MD depends on the IQ test/retest method. Use α = .01.

Source : Kanaya, T., Scullin, M. H., and Ceci, S. J. “The Flynn effect and U.S. Policies.” American Psychologist , Vol. 58, No. 10, Oct. 2003 ( Figure ). Copyright © 2003 by the American Psychological Association. Reprinted with permission.