If you read a lot but aren’t much into math you’ve probably heard the term standard deviation being tossed around, but didn’t know what it meant exactly.
A standard deviation is a way to describe where someone (or something) falls on a scale, and, more specifically, how wide the range actually is. Bell curves are often presented when talking about standard deviations, as they capture the same idea visually.
The real question is, how far is a given measurement from the average for a group?
Some distributions are very high curves with little variation within the population, while others are short and wide.
For example, the average height for adult men in the United States is about 70 inches (178 cm), with a standard deviation of around 3 in (8 cm). This means that most men (about 68 percent, assuming a normal distribution) have a height within 3 in (8 cm) of the mean (67–73 in/170–185 cm), one standard deviation, whereas almost all men (about 95%) have a height within 6 in (15 cm) of the mean (64–76 in/163–193 cm), 2 standard deviations. Three standard deviations account for 99.7% of the sample population being studied, assuming the distribution is normal (bell-shaped).