You remember the concept of standard deviation from high school statistics. It’s the measure of a set of data relative to a mean or a norm. It’s a way to quantify variation. A quick example, courtesy of Wikipedia, tells us that the average height for adult men in the United States is about 70 inches, with a standard deviation of around 3 inches. This means that most men (about 68%, assuming a normal distribution) have a height within 3 inches of the mean, or one standard deviation, and that almost all men (about 95%) have a height within 6 inches of the mean, or two standard deviations. If the standard deviation were zero, then all men would be exactly 70 inches, or 5ʹ10ʺ.