Standard Deviation

Measures spread of data around the mean.

s = \frac{\sum ( x _{i} - x ˉ ) ^{2}}{n - 1}

Variables

s: Sample standard deviation
x̄: Mean

Explanation

For an entire population divide by $n$ instead of $n - 1$ (giving $σ$ ). About 68% of normally distributed data lie within one standard deviation of the mean.

Example

Data 2, 4, 4, 4, 5, 5, 7, 9 has $σ = 2$ .

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Related formulas

Mean (Average)

The arithmetic average of a data set.