<P> To show that different batteries reflect the same g, one must administer several test batteries to the same individuals, extract g factors from each battery, and show that the factors are highly correlated . This can be done within a confirmatory factor analysis framework . Wendy Johnson and colleagues have published two such studies . The first found that the correlations between g factors extracted from three different batteries were . 99, . 99, and 1.00, supporting the hypothesis that g factors from different batteries are the same and that the identification of g is not dependent on the specific abilities assessed . The second study found that g factors derived from four of five test batteries correlated at between . 95--1.00, while the correlations ranged from . 79 to . 96 for the fifth battery, the Cattell Culture Fair Intelligence Test (the CFIT). They attributed the somewhat lower correlations with the CFIT battery to its lack of content diversity for it contains only matrix - type items, and interpreted the findings as supporting the contention that g factors derived from different test batteries are the same provided that the batteries are diverse enough . The results suggest that the same g can be consistently identified from different test batteries . </P> <P> The form of the population distribution of g is unknown, because g cannot be measured on a ratio scale . (The distributions of scores on typical IQ tests are roughly normal, but this is achieved by construction, i.e., by normalizing the raw scores .) It has been argued that there are nevertheless good reasons for supposing that g is normally distributed in the general population, at least within a range of ± 2 standard deviations from the mean . In particular, g can be thought of as a composite variable that reflects the additive effects of a large number of independent genetic and environmental influences, and such a variable should, according to the central limit theorem, follow a normal distribution . </P> <P> A number of researchers have suggested that the proportion of variation accounted for by g may not be uniform across all subgroups within a population . Spearman's law of diminishing returns (SLODR), also termed the cognitive ability differentiation hypothesis, predicts that the positive correlations among different cognitive abilities are weaker among more intelligent subgroups of individuals . More specifically, SLODR predicts that the g factor will account for a smaller proportion of individual differences in cognitive tests scores at higher scores on the g factor . </P> <P> SLODR was originally proposed by Charles Spearman, who reported that the average correlation between 12 cognitive ability tests was . 466 in 78 normal children, and . 782 in 22 "defective" children . Detterman and Daniel rediscovered this phenomenon in 1989 . They reported that for subtests of both the WAIS and the WISC, subtest intercorrelations decreased monotonically with ability group, ranging from approximately an average intercorrelation of . 7 among individuals with IQs less than 78 to . 4 among individuals with IQs greater than 122 . </P>

Is there a perfect way to measure intelligence