The standard deviation is the average amount of variability of a population or a sample. It is a measure of dispersion: low standard deviation means that values are located closer to the mean, while a larger standard deviation means that there is a higher dispersion. It is usually represented using $\sigma$ or $S$ and can be computed using: $ \sigma = \sqrt{ \frac {\sum{(x_i - \mu)^2}} {N} } $ Being $N$ the size of the population, $x_i$ each sample of the population and $\mu$ the [[Mean|mean]] of the population.