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<p>Time to get out my FAQ electrons. </p>
<p>All IQ tests have error of estimation, and no one brand of IQ test will yield the same score for the same individual on every occasion, nor will two different brands of IQ tests necessarily sort the same group of test-takers into the same rank order. Lewis Terman (the developer of the first widely used IQ test in the United States, the Stanford-Binet, the test that Marilyn vos Savant took and I took in childhood) noted that error of estimation in IQ scores increases as IQ scores are above the mean: </p>
<p>"The reader should not lose sight of the fact that a test with even a high reliability yields scores which have an appreciable probable error. The probable error in terms of mental age is of course larger with older than with young children because of the increasing spread of mental age as we go from younger to older groups. For this reason it has been customary to express the P.E. [probable error] of a Binet score in terms of I.Q., since the spread of Binet I.Q.'s is fairly constant from age to age. However, when our correlation arrays [between Form L and Form M] were plotted for separate age groups they were all discovered to be distinctly fan-shaped. Figure 3 is typical of the arrays at every age level.</p>
<p>“From Figure 3 [not shown here on CC, alas] it becomes clear that the probable error of an I.Q. score is not a constant amount, but a variable which increases as I.Q. increases. It has frequently been noted in the literature that gifted subjects show greater I.Q. fluctuation than do clinical cases with low I.Q.'s . . . . we now see that this trend is inherent in the I.Q. technique itself, and might have been predicted on logical grounds.” (Terman & Merrill, 1937, p. 44)</p>
<p>A slightly different issue is that anyone’s IQ can change over the course of life. For example, young people in the famous Lewis Terman longitudinal Genetic Studies of Genius (initial n=1,444 with n=643 in main study group) when tested at high school age (n=503) were found to have dropped 9 IQ points on average in Stanford-Binet IQ. More than two dozen children dropped by 15 IQ points and six by 25 points or more. Parents of those children reported no changes in their children or even that their children were getting brighter (Shurkin 1992, pp. 89-90). Terman observed a similar drop in IQ scores in his study group upon adult IQ testing (Shurkin 1992, pp. 147-150). Samuel R. Pinneau conducted a thorough review of the Berkeley Growth Study (1928-1946; initial n=61, n after eighteen years =40). Alice Moriarty was a Ph.D. researcher at the Menninger Foundation and describes in her book (1966) a number of case studies of longitudinal observations of children’s IQ. She observed several subjects whose childhood IQ varied markedly over the course of childhood, and develops hypotheses about why those IQ changes occurred. Anastasi and Urbina (1997, p. 328) point out that childhood IQ scores are poorest at predicting subsequent IQ scores when taken at preschool age. Change in scores over the course of childhood shows that there are powerful environmental effects on IQ (Anastasi & Urbina 1997, p. 327) or perhaps that IQ scores in childhood are not reliable estimates of a child’s scholastic ability. </p>
<p>Lewis Terman’s longitudinal study of high-IQ elementary-age pupils showed that many of those young people did not qualify as “gifted” on a subsequent test that Terman gave them at high school age. But he kept them in the study group anyway. An especially odd result of the Terman study is that Terman tested and rejected for inclusion in his study two children whose IQ scores were below his cut-off line who later went on to win Nobel prizes: William Shockley, who co-invented the transistor, and physicist Luis Alvarez. None of the children included in the study ever won a Nobel prize. The book by Shurkin I have just cited here is a good corrective on many misconceptions about IQ, and has an excellent section on attempts by one of Terman’s study associates to estimate–by extremely dubious methods that have never been validated–the IQ scores of historical persons. </p>
<p>REFERENCES</p>
<p>Anastasi, Anne & Urbina, Susana (1997). Psychological Testing. Upper Saddle River, NJ: Prentice Hall.</p>
<p>Deary, Ian J. (2000) Looking Down on Human Intelligence: From Psychometrics to the Brain. Oxford: Oxford University Press.</p>
<p>Howe, Michael J. A. (1998). Can IQ Change?. The Psychologist, February 1998 pages 69-72.</p>
<p>Pinneau, Samuel R. (1961). Changes in Intelligence Quotient Infancy to Maturity: New Insights from the Berkeley Growth Study with Implications for the Stanford-Binet Scales and Applications to Professional Practice. Boston: Houghton Mifflin. </p>
<p>Shurkin, Joel N. (1992). Terman’s Kids: The Groundbreaking Study of How the Gifted Grow Up. Boston: Little, Brown.</p>
<p>Terman, Lewis & Merrill, Maude (1937). Measuring Intelligence: A Guide to the Administration of the New Revised Stanford-Binet Tests of Intelligence. Boston: Houghton Mifflin.</p>
<p>Truch, Steve (1993). The WISC-III(R) Companion: A Guide to Interpretation and Educational Intervention. Austin, TX: Pro-Ed.</p>