The standard deviation (SD or s) is a measure of the width or spread of Gaussian/normal/bellcurve curves that approximately fit the distributions of intelligence, heights, weights, etc. The larger the value of s, the broader the curve will look.  68% of the population has an IQ lying within ±1 standard deviation (s) of the mean (average) IQ of 100. 96% of the population has an IQ lying between -2 s and +2 s. 99.6% of the population posseses an IQ within the range -3 s to +3 s, while 99.994% may be found within the range -4 s through 4 s.
    In the diagram below, s = 16 points of IQ. It may be seen from this curve that 98% of all individuals lie below an IQ that is 2 s  above the mean (average) IQ of 100 (96% between -2 s and +2 s (IQ = 68 through IQ = 132), plus 2% below an IQ of 68.
    A standard deviation of 15 is also popular on IQ tests. It's important to determine whether the standard deviation being used is 15 or 16 when assessing IQ-test results.