Many children who do well on other tests (IQ, achievement), seem to not score as high as expected on the CogAT. There are many things that could cause a bright child not to do well on a group test. So remember, it is just one day and one test. Some DC might do better on another type of test.
My DD got lucky and did well, but I'd mentally prepared for the worst because CogAT is so much trouble for so many bright kiddos around here. Still, DD's Q scores were far below her achievement scores in math, so this is a different kind of test. Honestly, though, her verbal abilities are probably her strength and her scores were quite high there - so maybe the test was right about her. Still...just one test! If you think your DD is bright, she probably is!
