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javascript/snippets/standard-deviation.md

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+---
+title: Standard deviation
+type: snippet
+tags: [math]
+cover: interior-16
+dateModified: 2020-10-22T20:24:30+03:00
+---
+
+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.
+
+```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((val - mean) ** 2), [])
+      .reduce((acc, val) => acc + val, 0) /
+      (arr.length - (usePopulation ? 0 : 1))
+  );
+};
+```
+
+```js
+standardDeviation([10, 2, 38, 23, 38, 23, 21]); // 13.284434142114991 (sample)
+standardDeviation([10, 2, 38, 23, 38, 23, 21], true);
+// 12.29899614287479 (population)
+```