code: @Override
      public double getStandardDeviation() {
          double stdDev = Double.NaN;
          if (getN() > 0) {
              if (getN() > 1) {
                  stdDev = FastMath.sqrt(getVariance());
              } else {
                  stdDev = 0.0;
              }
          }
          return stdDev;
      }
docs: Returns the standard deviation of the values that have been added.
  
  <p>Double.NaN is returned if no values have been added.
  
  @return the standard deviation