Cognitive Ability: G Factor

‘General Ability’ and ‘Ability in General’

It is important to distinguish between general ability (or the g factor), on the one hand, and what can be called ability in general, on the other. The latter refers to the sum (or average) of the scores on a collection of different subtests, such as the Stanford-Binet and the Wechsler batteries, and many other heterogeneous tests of ‘intelligence’. The total score or Full Scale IQ on such tests is based on an arbitrary selection of a number of diverse tests. The g factor, however, in a linear decomposition of the total variance into uncorrelated components or factors, reflects only the source of variance that is common to all of the different ability measures represented by the various subtests of a cognitive test battery. Hence the simple sum of the subtest standardized scores on a test battery and the factor scores obtained from the g loadings of various subtests are not necessarily the same and may even be quite different. Typically, however, in the most widely used and broadly valid standardized test batteries [Page 212]the sum of the subtest scores (e.g. the full scale IQ) and the g factor scores are very highly correlated. Therefore the labour of calculating factor scores has little justification for the practical use of tests; the total standardized score, or IQ, is a fair substitute for the estimated g factor score.

Test Construction for the Measurement of g

The psychometric procedures for constructing an IQ test are intended to yield high practical validity for predicting many different outcomes involving cognitive abilities, such as scholastic achievement, college grade-point average, job performance, occupational level, and success in various armed services training programmes. This aim of IQ test construction and the psychometric means for achieving it inevitably results in the production of highly g loaded IQ scores, even when factor analysis is not used and the test designers have no interest in maximizing the test's overall g saturation per se.

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