Mean

The mean is the balance point of a distribution, the value where the data would balance if placed on a seesaw. For example, the mean of 4, 8, and 9 is $(4 + 8 + 9) / 3 = 7$. The sample mean is written $\bar{x} = \frac{1}{n}\sum x_i$, where $\sum x_i$ (the sum of the values) is divided by $n$ (the number of values). Because every value contributes, the mean is sensitive to outliers and skew.