The standard deviation of a random variable is defined as the positive square root of the
Variance. Standard deviation is a measure of the variability of the variable about the mean. If a variable is normally-distributed (in other words, its distribution follows the standard bell-shaped curve), 68.3% of the time its value will fall within one standard deviation of the mean, 95.4% within two standard deviations, and 99.7% within 3 standard deviations.