The goal of most intelligence tests is to measure g, the general intelligence factor. Good intelligence tests are reliable, meaning that they are consistent over time, and also demonstrate construct validity, meaning that they actually measure intelligence rather than something else. Because intelligence is such an important individual difference dimension, psychologists have invested substantial effort in creating and improving measures of intelligence, and these tests are now the most accurate of all psychological tests. In fact, the ability to accurately assess intelligence is one of the most important contributions of psychology to everyday public life.
Intelligence changes with age. A 3-year-old who could accurately multiply 183 by 39 would certainly be intelligent, but a 25-year-old who could not do so would be seen as unintelligent. Thus understanding intelligence requires that we know the norms or standards in a given population of people at a given age. Thestandardization of a test involves giving it to a large number of peopleat different ages and computingtheaveragescoreon thetest at each agelevel.
It is important that intelligence tests be standardized on a regular basis, because the overall level of intelligence in a population may change over time. The Flynn effect refers to the observation that scores on intelligencetests worldwidehaveincreased substantiallyover thepastdecades (Flynn, 1999). 1 Although the increase varies somewhat from country to country, the average increase is about 3 IQ points every 10 years. There are many explanations for the Flynn effect, including better nutrition, increased access to information, and more familiarity with multiple-choice tests (Neisser, 1998). 2 But whether people are actually getting smarter is debatable (Neisser, 1997). 3
Once the standardization has been accomplished, we have a picture of the average abilities of people at different ages and can calculate a person’smental age, which is theageat which a person is performing intellectually. If we compare the mental age of a person to the person’s chronological age, the result is the intelligence quotient (IQ), a measureofintelligencethat is adjusted for age. A simple way to calculate IQ is by using the following formula:
IQ = mental age ÷ chronological age × 100.
Thus a 10-year-old child who does as well as the average 10-year-old child has an IQ of 100 (10 ÷ 10 × 100), whereas an 8-year-old child who does as well as the average 10-year-old child would have an IQ of 125 (10 ÷ 8 × 100). Most modern intelligence tests are based the relative position of a person’s score among people of the same age, rather than on the basis of this formula, but the idea of an intelligence “ratio” or “quotient” provides a good description of the score’s meaning.
A number of scales are based on the IQ. TheWechsler Adult lntelligence Scale (WAIS) is the most widelyused intelligencetest for adults (Watkins, Campbell, Nieberding, & Hallmark, 1995). 4The current version of the WAIS, the WAIS-IV, was standardized on 2,200 people ranging from 16 to 90 years of age. It consists of 15 different tasks, each designed to assess intelligence, including working memory, arithmetic ability, spatial ability, and general knowledge about the world (see Figure 9.4 "Sample Items From the Wechsler Adult Intelligence Scale (WAIS)"). The WAIS-IV yields scores on four domains: verbal, perceptual, working memory, and processing speed. The reliability of the test is high (more than 0.95), and it shows substantial construct validity. The WAIS-IV is correlated highly with other IQ tests such as the Stanford-Binet, as well as with criteria of academic and life success, including college grades, measures of work performance, and occupational level. It also shows significant correlations with measures of everyday functioning among the mentally retarded.
The Wechsler scale has also been adapted for preschool children in the form of the Wechsler Primaryand Preschool Scaleof Intelligence(WPPSI-III)and for older children and adolescents in the form of the Wechsler IntelligenceScaleforChildren (WISC-IV).
The intelligence tests that you may be most familiar with are aptitudetests, which are designed to measure one’s ability to perform a given task, for instance, to do well in college or in postgraduate training. Most U.S. colleges and universities require students to take the Scholastic Assessment Test (SAT) or the American College Test (ACT), and postgraduate schools require the Graduate Record Examination (GRE), Medical College Admissions Test (MCAT), or the Law School Admission Test (LSAT). These tests are useful for selecting students because they predict success in the programs that they are designed for, particularly in the first year of the program (Kuncel, Hezlett, & Ones, 2010). 5 These aptitude tests also measure, in part, intelligence. Frey and Detterman (2004) 6found that the SAT correlated highly (between about r = .7 and r = .8) with standard measures of intelligence.
Intelligence tests are also used by industrial and organizational psychologists in the process of personnel selection. Personnel selection is theuseof structured tests to select peoplewho are likelyto perform well at given jobs(Schmidt & Hunter, 1998). 7The psychologists begin by conducting a job analysis in which they determine what knowledge, skills, abilities, and personal characteristics (KSAPs) are required for a given job. This is normally accomplished by surveying and/or interviewing current workers and their supervisors. Based on the results of the job analysis, the psychologists choose selection methods that are most likely to be predictive of job performance. Measures include tests of cognitive and physical ability and job knowledge tests, as well as measures of IQ and personality.
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