Merge pull request #68 from atomiks/patch-3

Create standard-deviation.md

snippets/standard-deviation.md

@@ -0,0 +1,17 @@
+### Standard deviation
+
+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))
+  );
+ }
+// standardDeviation([10,2,38,23,38,23,21]) -> 13.284434142114991 (sample)
+// standardDeviation([10,2,38,23,38,23,21], true) -> 12.29899614287479 (population)
+```