The Normal Distribution of Scores for a Standardized Intelligence Test

This distribution of intelligence test scores in the population is standardized to have a mean of 100, so half of the population scores higher than 100, and half score lower than 100. The standard deviation (roughly, the average number of points away from the average) is approximately 15. Note the percentages in the curve. Nearly 68% of the population’s scores fall within one standard deviation from the mean (between 85 and 115) and approximately 95% of scores fall within two standard deviations of the mean (between 70 and 130).