(One might also interpret this as saying that the residual uncertainty in "true g score" is a good chunk of an SD even after an individual is carefully tested using WAIS or WISC; personally I don't think "g" is much better defined than at this level of accuracy.)
I am very jet-lagged right now, so I hope what I wrote makes sense :-)
Verbal and non-verbal intelligence changes in the teenage brain (Nature):
... Our participants were 33 healthy and neurologically normal adolescents with a deliberately wide and heterogeneous mix of abilities (see Supplementary Information for details and the implications of our sampling for the generalizability of our conclusions). They were first tested in 2004 (‘time 1’) when they were 12–16 yr old (mean, 14.1 yr). Testing was repeated in 2007/2008 (‘time 2’) when the same indivi- duals were 15–20 yr old (mean, 17.7 yr). See Table 1 for further details of the participants. During the intervening years, there were no testing sessions, and participants and their parents had no knowledge that they would be invited back for further testing. On both test occasions, each participant had a structural brain scan using magnetic resonance imaging (MRI) and had their IQ measured using the Wechsler Intelligence Scale for Children (WISC-III) at time 1 and the Wechsler Adult Intelligence Scale (WAIS-III) at time 2 (see Supplementary Information for details). These two widely used, age-appropriate assess- ments5 produce strongly correlated results at a given time point, con- sistent with them measuring highly similar constructs6. Scores on individual subtests are standardized against age-specific norms and then grouped to produce separate measures of verbal IQ (VIQ) and performance IQ (PIQ), with VIQ encompassing those tests most related to verbal skills and PIQ being more independent of verbal skills. Nevertheless, VIQ and PIQ scores are very significantly correlated with each other across participants: in our sample, the correlations between VIQ and PIQ were r50.51 at time 1 and r50.55 at time 2 (in both cases, n 5 33; P , 0.01). Full-scale IQ (FSIQ) is the composite of VIQ and PIQ and is regarded as the best measure of general intellectual capacity (the g factor) that has previously been shown to correlate with brain size and cortical thickness in a wide variety of frontal, parietal and temporal brain regions7,8.
The wide range of abilities in our sample was confirmed as follows: FSIQ ranged from 77 to 135 at time 1 and from 87 to 143 at time 2, with averages of 112 and 113 at times 1 and 2, respectively, and a tight correlation across testing points (r 5 0.79; P , 0.001). Our interest was in the considerable variation observed between testing points at the individual level, which ranged from 220 to 123 for VIQ, 218 to 117 for PIQ and 218 to 121 for FSIQ. Even if the extreme values of the published 90% confidence intervals are used on both occasions, 39% of the sample showed a clear change in VIQ, 21% in PIQ and 33% in FSIQ. In terms of the overall distribution, 21% of our sample showed a shift of at least one population standard deviation (15) in the VIQ measure, and 18% in the PIQ measure. However, only one participant had a shift of this magnitude in both measures, and, for that particip- ant, one measure showed an increase and the other a decrease. This pattern is reflected in the absence of a significant correlation between the change in VIQ and the change in PIQ. The independence of changes in these two measures allows us to investigate the effect of each without confounding influences from the other.
... Using regression analysis, we studied the brain changes associated with a change in VIQ, PIQ or FSIQ (see Methods Summary for details). The results (Fig. 1) showed that changes in VIQ were positively corre- lated with changes in grey matter density (and volume) in a region of the left motor cortex that is activated by the articulation of speech10. Conversely, changes in PIQ were positively correlated with grey matter density in the anterior cerebellum (lobule IV), which is associated with motor movements of the hand