code: public double stdDev() {
          double avg = avgRate();
          double sum = 0.0;
          for (Sample s : samples) {
              sum += Math.pow((double) s.rate() - avg, 2);
          }
          double variance = sum / (double) samples.size();
          return Math.sqrt(variance);
      }
docs: Returns the standard deviation of the message rates in the SampleGroup.
  
  @return the standard deviation of the message rates in the SampleGroup