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Update standard-deviation.md

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Angelos Chalaris
2017-12-14 11:29:20 +02:00
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### Standard deviation
Use `Array.reduce()` to calculate the mean of the values, the variance of the values, and the sum of the variance
of the values to determine the standard deviation of an array of numbers.
Since there are two types of standard deviation, population and sample, you can use a flag to switch to population (sample is default).
Use `Array.reduce()` to calculate the mean, variance and the sum of the variance of the values, the variance of the values, then
determine the standard deviation.
You can omit the second argument to get the sample standard deviation or set it to `true` to get the population standard deviation.
```js
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(Math.pow(val - mean, 2)), [])
.reduce((acc, val) => acc + val, 0)
/ (arr.length - (usePopulation ? 0 : 1))
.reduce((acc, val) => acc + val, 0) / (arr.length - (usePopulation ? 0 : 1))
);
}
// standardDeviation([10,2,38,23,38,23,21]) -> 13.284434142114991 (sample)