Variation in a group or population is usually described with reference to one or more basic statistical functions. The mean or average of a group or distribution is the most common statistic used. It is calculated simply by adding all of the observations and dividing by the number of observations. The mean is sometimes referred to as the expected value since it is a measure of central tendenancy and in one single oberservation, represents all of the different observations in the population. All of the observations, when taken together, will be closer to the mean than any other single number.
