The arithmetic mean is often known simply as the mean. It is an average, a measure of the centre of a set of data. The arithmetic mean is calculated by adding up all the values and dividing the sum by the total number of values. Arithmetic mean or simply the mean of a variable is defined as the sum of the observations divided by the number of observations.
The arithmetic mean is the simplest calculation to determine the average return. The average return on an asset, observed over a long period of time, is often used as the expected return for future years. Although the future return is unknown and cannot be predicted with great accuracy, the historical average return is as good a guess as any of what the return will be in the future.