http://www.hoagiesgifted.org/tests_tell_us.htm "Group intelligence tests are commonly used as screening measures, to see if the child should move to a full gifted assessment. They are commonly administered by teachers. Group tests are generally normed on populations of all children, with relatively few gifted children among the mix. When taking group intelligence tests, gifted kids often "over-think" the questions, and perhaps make wrong selections. And since there's no individual tester to clarify unusual answers, the gifted kids often score lower on group intelligence tests."
Also, this, though it mentions OLSAT not CoGat. I imagine they're similar but I'm not an expert:
"The most common group intelligence tests, OLSAT and CogAT, are used in districts and programs across the country. Notable gifted professionals recommend them for screening potentially gifted children. However, a small study noted a potential problem with the OLSAT and very gifted children. While the correlation between group and individual intelligence tests is quite high for average scores, in this study that correlation almost disappeared for gifted scores. This means that while an average child will score very similarly on a group IQ test and an individual IQ test, a gifted child may not score similarly at all. And the study suggests that this group test may even result in a negative correlation for some gifted children: the more gifted the child, the lower the group ability test score! ["Investigations of the Otis-Lennon School Ability Test to Predict WISC-R Full Scale IQ for Referred Children" by Anna H. Avant and Marcia R. O'Neal, University of Alabama, Nov. 1986, ED286883] Though this study is no longer available from AskERIC, it can be obtained on microfiche from most education university libraries."
