## Formulae for the standard deviation

Whilst it is not necessary to learn the formula for calculating the standard deviation, there may be times when you wish to include it in a report or dissertation.

The standard deviation of an entire population is known as **σ** (sigma) and is calculated using:

Where **x** represents each value in the population, **μ** is the mean value of the population, **Σ** is the summation (or total), and **N** is the number of values in the population.

The standard deviation of a sample is known as **S** and is calculated using:

Where **x** represents each value in the population, **x** is the mean value of the sample, **Σ** is the summation (or total), and **n-1** is the number of values in the sample minus 1.