For what it's worth, DD6 scored 139 on the CogAT, but only 120 on WISC-IV. I tend to think her IQ is somewhere in the middle, and the more I work with her, the more I believe she really is gifted.
I think that for her, the CogAT tested exactly what she's good at. Math is a real strong point of hers, and it's not tested at all on the WISC (at least not on the parts she took.) On the CogAT she got a perfect in Quantitative, and at home I see that she can take one mathematical concept (like time on a clock) and without any prompting apply it to another area (like fractions). This is in stark contrast to DD8, who despite learning how to round to the tens place and hundreds place, cannot fathom how that would help her round to the thousands place! DD6 is also a strong reader. We gave her the DORA and she maxed out many of the subtests (except spelling - there she is grade level), scoring an average of about the 6th grade level while in 1st grade.
On the other and, I think the free response on the WISC was a bad fit for DD6. She's a perfectionist with possible OCD, and she will not tell you an answer unless she's sure that it's right! Even with me she's reluctant to guess, so I can imagine it would be worse with a stranger since she's also terribly shy.
I'm sorry - I did not mean to write so much about my family! I don't think any test of IQ is perfect, and the CogAT doesn't even claim to be an IQ test. However, I think a good score on ANY test is good news! A lot of parents here tend to downplay the CogAT, because their gifted kids sometimes don't do as well on it, but in reality it might be a great measure of predicted school achievement.
