code: function variance(x) {
          // The variance of no numbers is null
          if (x.length === 0) return null;
  
          var mean_value = mean(x),
              deviations = [];
  
          // Make a list of squared deviations from the mean.
          for (var i = 0; i < x.length; i++) {
              deviations.push(Math.pow(x[i] - mean_value, 2));
          }
  
          // Find the mean value of that list
          return mean(deviations);
      }
docs: # [variance](http://en.wikipedia.org/wiki/Variance)  is the sum of squared deviations from the mean  depends on `mean()`