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A single item that represents a group. For example one student might be chosen as an average student to represent their grade. In mathematics the average is a number that represents a set of data.
The mean or arithmetic mean, the median, and the mode are the most commonly used averages.

Arithmetic mean or Mean or Average is perhaps the best known measure of central tendency (of the location) for interval and ratio scale variables.
Arithmetic mean is calculated by summing a set of data values and dividing by the number of values.

- x   = 1 n ∑ xi .

Arithmetic mean is sensitive to the outlier points, but the median is not.