rows · IQ Percentile and Rarity Chart. These are IQs, their percentiles, and rarity on a 15 SD (e.g. . On many tests, a score of is considered the average IQ. Sixty-eight percent of scores fall within one standard deviation of the mean (that is, between 85 and ). That means that nearly 70% of all people score within plus or minus 15 points of the average score.
In his system the IQ of those whose score equalled the mean of the age group was This way the IQ of the average adult would be , just like the IQ of the average child in the original system. He used the statistical properties of the normal distribution to assign IQ scores based on the extent of the contemporaries one outscored.