As a common format, these tables use a Gamma scale* IQ with s.d.=15.53, percentile 99.5 defining 140 IQ.
This reasonably approximates the most frequently-encountered SDs between fifteen and sixteen points. Among other advantages, the two most common "confidence interval" benchmarks fall at 130 and 140 IQ, respectively. In many Gifted and Talented educational programs, the 130 IQ level is regarded as a threshold of intellectual giftedness. As a rule of thumb, ten-point differences are likely to represent noticeable differences in mental ability among individuals, while differences of twenty or more points typically present some barriers to effective communication. One could debate that point, considering that perceived differences also arise from differing patterns of individual strengths and weaknesses, even among those sharing similar full-scale IQ scores. Those are the very differences which contribute such intriguing depth to intragroup exchanges.

It is reasonable to suggest that there may be some understatement of the " true " scores of some members owing to the ceiling limitations of several of the supervised tests used for qualification. This is particularly a concern with scores derived from the GRE and similar academic measures. WAIS and the Cattell Culture Fair are probably the only tests covering the full range of scores indicated in the table. In fact, ongoing review of the available tests suggest that scores above the 99.99 percentile are rarely encountered or attainable in the realm of standardized testing for adults ... and certainly questionable on experimental tests.

* The standard deviation and mean of the Otis-Gamma test are, for practical purposes, the same as those used here. Besides, it evokes a smile in the association with Huxley's famed novel. Irresistible!