Here is a slightly harder, real-life example: The average height for grown men in the United States is 70", with a standard deviation of 3". A standard deviation of 3” means that most men (about 68%, assuming a normal distribution) have a height 3" taller to 3” shorter than the average (67"–73") — one standard deviation. Almost all men (about 95%) have a height 6” taller to 6” shorter than the average (64"–76") — two standard deviations. Three standard deviations include all the numbers for 99.7% of the sample population being studied. This is true if the distribution is normal (bell-shaped).