Kevin McGrew and I wrote a position paper about the factors that influence the inaccuracy of averaged pseudo-composite scores compared to real composite scores. Averaged pseudo-composites are simple averages of test scores. For example, if a person scores 6 on WISC-IV Digit Span and 4 on WISC-IV Letter-Number Sequencing, an averaged pseudo-composite score measuring “Working Memory” would be 5, in the index score metric, 75. In truth, the real composite score should be lower than 75, depending on the correlation between the two subtests.
The paper can be downloaded here.
Disclaimer: The paper is not yet peer-reviewed.