Arithmetic Mean

The arithmetic mean, often called simply the average, is the sum of a collection of numbers divided by how many numbers are in the collection. It is usually written with a bar over the variable, such as x-bar, and is called the population mean when computed over an entire population or the sample mean when computed over a subset. The arithmetic mean differs from the median, which is the middle value of a data set rather than the sum divided by the count, and it is not a robust statistic because a single large outlier can pull it far from the bulk of the data. It is used throughout mathematics, statistics, economics and other fields wherever a single representative value for a set of numbers is needed.