Most IQ tests have ceilings of 155-160 or even less. Some tests given to our young children, supposedly resulting in an IQ score, have ceilings as low as 135. It means if Barb and Bill both took a test that has a ceiling of 135, and both scored between 130 and 135, you might be fooled into thinking that both Barb and Bill are moderately gifted. If both later take a test with a much higher ceiling, Barb might again score around 135, proving that her original score was fairly accurate for her. Although his initial score made Bill look only moderately gifted, his score was depressed by the ceiling of the test, and in fact Bill is exceptionally gifted.
Gifted children tend to think differently and learn more quickly than their peers.
Again, you will see the difference, in many of the same ways. Now compare the highly gifted child to the normal child, and you face a chasm that by the end of elementary school may place these two children as much as 5 years apart in mental age. There's another way to look at it. The difference between the exceptionally gifted and the average child is the same as the difference between the average child and the mentally handicapped child of IQ 40. Ceiling effects may be particularly misleading in cases where the child has widely disparate abilities.