normal distribution
a theoretical distribution in which values pile up in the center at the mean and fall off into tails at either end. When plotted, it gives the familiar bell-shaped curve expected when variation about the mean value is random. The normal distribution has several primary characteristics: It is symmetrical, it has both upper and lower asymptotes, and its mean, median, and mode are the same value. Perhaps most important, however, fixed proportions of values fall within defined sections of the distribution. For example, 34.13% of values fall between the mean and one standard deviation
above the mean, and a corresponding 34.13% of values fall between the mean of the distribution and one standard deviation below the mean. Many statistical models are based on the assumption that data follow a normal distribution. For example, it is reasonable to expect that human height follows a normal distribution with a mean of 5 feet and several inches, such that very few adults are less than 3 feet or greater than 7 feet tall. Also called Gaussian distribution; normal curve.