code: public double getPopulationVariance() {
        if (getCount() <= 0) {
           return Double.NaN;
        } else if (getCount() == 1) {
           return 0d;
        }
        return Math.abs(getSumOfSquares() - getAverage() * getSum()) / getCount();
     }
docs: Gets the population variance.
  
  @return the population variance