# What is the formula to calculate the standard deviation of a sample?

$\sigma = \sqrt{\frac{1}{N} \sum {\left({x}_{i} - \mu\right)}^{2}}$
$\mu$ is the average of all ${x}_{i}$. The first deviation, sum of ${x}_{i} - \mu$ is by definition zero. The second deviation is the first to give us some information on the spread of values around the mean. Note it's a symmetric measure, it does not distinguish between values above or below the mean. Third deviation does. It's called the skewness.