The mean, also called the “average” or the “arithmetic mean”, is often used as a summary of the center of a set of numbers, such as the ages of all the participants in a clinical trial. It is calculated by adding all the numbers together and then dividing by how many of them there are. It is most useful when the set of numbers approximately follow a Normal distribution. In other cases, such as for very skewed distributions or when there are large outliers, other summary measures such as the median or geometric mean may more accurately convey the “center” of the set of numbers.