code: @Override
      public double getPopulationVariance() {
          Variance populationVariance = new Variance(_getSecondMoment());
          populationVariance.setBiasCorrected(false);
          return populationVariance.getResult();
      }
docs: Returns the <a href="http://en.wikibooks.org/wiki/Statistics/Summary/Variance">population
  variance</a> of the values that have been added.
  
  <p>Double.NaN is returned if no values have been added.
  
  @return the population variance