code: public synchronized double variance()
      {
          long n = 0;
          double mean = 0;
          double s = 0.0;
  
          for (double x : m_times.m_values)
          {
              n++;
              double delta = x - mean;
              mean += delta / n;
              s += delta * (x - mean);
          }
  
          return (s / n);
      }
docs: Returns the variance of all StopWatches recorded. NB: This
  call causes all of the StopWatches to be traversed, which makes it fairly slow.
  
  @return The variance.