docs: public double stdev() {
          double m = mean();
  
          double total = 0;
  
          final int N = getNumElements();
          if( N <= 1 )
              throw new IllegalArgumentException("There must be more than one element to compute stdev");
  
  
          for( int i = 0; i < N; i++ ) {
              double x = get(i);
  
              total += (x - m)*(x - m);
          }
  
          total /= (N-1);
  
          return Math.sqrt(total);
      }