Create standard-deviation.md

http://www.calculator.net/standard-deviation-calculator.html

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snippets/standard-deviation.md

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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.
+
+NOTE: This is **population standard deviation**. Use `/ (arr.length - 1)` at the end to 
+calculate **sample standard deviation**.
+
+```js
+const standardDeviation = (arr, val) =>
+  Math.sqrt(
+    arr.reduce((acc, val) => acc.concat(Math.pow(val - arr.reduce((acc, val) => acc + val, 0) / arr.length, 2)), [])
+       .reduce((acc, val) => acc + val, 0) 
+    / arr.length
+  );
+```