I administered the WISC–IV UK to a student who scored 65 on each of the VCI, PRI, WMI and PSI indexes, but his FSIQ was 57. Shouldn’t it be 65?
Many people find this result counterintuitive, but it is correct. First, consider that the FSIQ is used to predict the student’s true intelligence and does not correlate perfectly with it. Then consider that the index scores are composed of fewer subtests than the full scale IQ score and do not correlate perfectly with the FSIQ. In this case, if the student’s true IQ is 57, then his or her index scores should be higher than 57 due to the effect of regression toward the mean. On the other end of the continuum, the opposite is true. If a student’s FSIQ is 147, there is a greater probability that his or her index scores will be lower than the FSIQ. This effect can be found in the composite score norms tables of many tests of cognitive ability, though the strength of the effect depends on several factors, including the number of subtests entering the composite, and the distance of the subtest scores from the mean, and the correlation among those subtests. When a composite is made up of mor
