Bingo. There is finally an excellent, relatively brief, explanation of the phenomena of why full scale IQ scores often diverge markedly from the arithmetic average of the component index or subtest scores.
This composite score extremity effect
) has been well known by users of the WJ batteries. Why....because the WJ has placed the global IQ composite and the individual tests on the same scale (M=100; SD=15). In contrast, most other cognitive ability batteries (e.g., Wechslers) have the individual test scores on a different scale (M=10; SD=3). The use of different scales has hidden this statistical score effect from users. It has always been present. I have written about this many times. One can revisit my latest post on this issue here
Now that the WISC-V measures a broader array of cognitive abilities (e.g., 5 index scores), users have been asking the same "why does the total IQ score not equal the average of the index scores?" Why? Because the five index scores are on the same scale as the full scale IQ score...and thus this composite score extremity effect is not hidden. A recent thread on the NASP Community Exchange
provides examples of psychologists wondering about this funky test score issue (click here to read
As per usual, Dr. Schneider has provided intuitive explanations of this score effect, and for those who want more, extremely well written technical explanations.
The WJ IV ASB 7
can be downloaded by clicking here
. Although written in the context of the WJ IV, this ASB is relevant to all intelligence test batteries that provide a global IQ score that is the sum of part scores.
Kudos to Dr. Schneider.
Click on image to enlarge