Bell Curve

580 Appendix 3 Appendix 3 5 8 1the internal psychometric properties of the AFQT and show that theAFQT is one of the most highly g-loaded mental tests in current use. Itseems to do what a good IQ test is supposed to do-tap into a generalfactor rather than specific bits of learning or skill-as well as or betterthan its competitors. Second, we examine the correlation between theAFQT and other IQ tests, and show that the AFQT is more highly correlatedwith a wide range of other mental tests than those other mentaltests are with each other. On both counts, the AFQT qualifies notjust as an IQ test, but one of the better ones psychometrically.Psychometric Characteristics of the ASVABLet us begin by considering the larger test from which the AFQT is computed,the ASVAB (Armed Services Vocational Aptitude Battery),taken every year by between a half million and a million young adultswho are applying for entry into one of the armed services. The ASVARhas ten subtests, spanning a range from test items that could appearequally well on standard tests of intelligence to items testing knowledgeof automobile repair and electronics.'" Scores on the subtests determinewhether the applicant will be accepted by his chosen branch of service;for those accepted, the scores are later used for the placement of enlistedpersonnel into military occupations. How well or poorly a person performsin military occupational training schools, and also how well hedoes on the job, can therefore be evaluated against the scores earned ona battery of standardized tests.The ten subtests of ASVAB can be paired off into forty-five correlations.Of the forty-five, the three highest correlations in a large studyof enlisted personnel were between Word Knowledge and General Science,Word Knowledge and Paragraph Completion, and, highest of all,between Mathematics Knowledge and Arithmetic Reasoning.* Correlationsabove .8, as these were, are in the range observed between differentIQ tests, which are frankly constructed to measure the sameattribute. To see them arising between tests of such different subjectmatter should alert us to some deeper level of mental functioning. Thethree lowest correlations, none lower than .22, were between CodingSpeed and Mechanical Comprehension, Numerical Operations andAutolShop Information, and, lowest of all, between Coding Speed andAutomobile/Shop Information. Between those extremes, there wererather large correlations between Paragraph Completion and GeneralScience and between Word Knowledge and Electronics Informationbut only moderate correlations between Electronics Information andCoding Speed and between Mathematics Knowledge and AutomobilelShopInformation. Thirty-six of the forty-five correlations wereabove -5.Psychometrics approaches a table of correlations with one or anotherof its methods for factor analysis. Factor analysis (or other mathematicalprocedures that go under other names) extracts the factors"' that accountfor the observed pattern of subtest scores. The basic idea is thatscores on any pair of tests are correlated to the extent that the tests measuresomething in common: If they test traits in common, they are correlated,and if not, not. Factor analysis tells how many differentunderlying factors are necessary to account for the observed correlationsbetween them. If, for example, the subtest scores were totally uncorrelated,it would take ten independent and equally significant factors, onefor each subtest by itself. With each test drawing on its own unique factor,the forty-five correlations would all be zeros. At the other extreme,if the subtests measured precisely the same thing down to the very smallestdetail, then all the correlations among scores on the subtests couldbe explained by a single factor-that thing which all the subtests preciselymeasured-and the correlations would all be ones. Neither extremedescribes the actuality, but for measures of intellectualperformance, one large factor comes closer than many small ones. Thisis not the place to dwell on mathematical details except to note that,contrary to claims in nontechnical works,4 the conclusions we drawabout general intelligence do not depend on the particular method ofanalysis used.5For the ASVAR, 64 percent of the variance among the ten subtestscores is accounted for by a single factor, g. A second factor accounts foranother 13 percent. With three inferred factors, 82 percent of the varianceis accounted for.'" l e intercorrelations indicate that people dovary importantly in some single, underlying trait and that those variationsaffect how they do on every test. Nor is the predominance of g afortuitous result of the particular subtests in ASVAB. The air force's aptitudetest for prospective officers, the AFOQT (Air Force Officer QualifyingTest) similarly has g as its major source of individual variation.'Indeed, all broad-gauged test batteries of cognitive ability have g as theirmajor source of variation among the scores people get.R