Obviously, we are focused on IQ tests as they relate to the gifted. However...
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The IQ tests are designed to make the finest distinctions for most of the children taking the test (the 95% of children who fall between 70 and 130.) We are asking a test to make fine distinctions with a relatively blunt tool when we are looking at the extremes.
http://www.sengifted.org/articles_directorscorner/Webb_Nov06.shtml