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IQ Test Reliability

The unreliability of IQ tests has been proved by numerous researchers. The scores may vary by as much as 15 points from one test to another,1 while emotional tension, anxiety, and unfamiliarity with the testing process can greatly affect test performance.2 In addition, Gould described the biasing effect that tester attitudes, qualifications, and instructions can have on testing.3

In one study, ninety-nine school psychologists independently scored an IQ test from identical records, and came up with IQ's ranging from 63 to 117 for the same person.4 In another study, Ysseldyke et al. examined the extent to which professionals were able to differentiate learning-disabled students from ordinary low achievers by examining patterns of scores on psychometric measures. Subjects were 65 school psychologists, 38 special-education teachers, and a “naive” group of 21 university students enrolled in programs unrelated to education or psychology. Provided with forms containing information on 41 test or subtest scores (including the WISC-R IQ test) of nine school-identified LD students and nine non-LD students, judges were instructed to indicate which students they believed were learning disabled and which were non-learning disabled. The school psychologists and special-education teachers were able to differentiate between LD students and low achievers with only 50 percent accuracy. The naive judges, who had never had more than an introductory course in education or psychology, evidenced a 75 percent hit rate!5

Article sources:
1.) Smith, C. R., Learning Disabilities: The Interaction of Learner, Task, and Setting (Boston: Allyn and Bacon, 1991), 63.
2.) Tyler, cited in A. Anastasi, (ed.), Testing Problems in Perspective (Washington DC: American Council on Education, 1966).
3.) Gould, S. J., The Mismeasure of Man (New York: W. W. Norton, 1981), 151-152, cited in R. L. Osgood, “Intelligence testing and the field of learning disabilities: A historical and critical perspective,” Learning Disability Quarterly, 1984, vol. 7, 343-348.
4.) Cited in J. Sattler, Assessment of Children's Intelligences and Special Abilities (Boston: Allyn & Bacon, 1982), 60.
5.) Epps, S., Ysseldyke, J. E., & McGue, M., “'I know one when I see one' — Differentiating LD and non-LD students,” Learning Disability Quarterly, 1984, vol. 7, 89-101.