docs: public static double calPairwiseVariance(long count1, double sum1,
              double variance1, long count2, double sum2, double variance2) {
          return (variance1 * (count1 - 1) + variance2 * (count2 - 1)
                  + pow(sum2 / count2 - sum1 / count1, 2) * count1 * count2
                  / (count1 + count2))
                  / (count1 + count2 - 1);
      }