# Standard deviation

JavaScript, Math · Oct 22, 2020

Calculates the standard deviation of an array of numbers.

- Use
`Array.prototype.reduce()`

to calculate the mean, variance and the sum of the variance of the values and determine the standard deviation. - Omit the second argument,
`usePopulation`

, to get the sample standard deviation or set it to`true`

to get the population standard deviation.

```
const standardDeviation = (arr, usePopulation = false) => {
const mean = arr.reduce((acc, val) => acc + val, 0) / arr.length;
return Math.sqrt(
arr
.reduce((acc, val) => acc.concat((val - mean) ** 2), [])
.reduce((acc, val) => acc + val, 0) /
(arr.length - (usePopulation ? 0 : 1))
);
};
```

```
standardDeviation([10, 2, 38, 23, 38, 23, 21]); // 13.284434142114991 (sample)
standardDeviation([10, 2, 38, 23, 38, 23, 21], true);
// 12.29899614287479 (population)
```