code: public final double sampleSigma(int n)
    {
      synchronized (_lock) {
        long count = _count.get();
        long lastCount = _lastStdCount;
        _lastStdCount = count;
  
        double sum = _sum.get();
        double lastSum = _lastStdSum;
        _lastStdSum = sum;
  
        double sumSquare = _sumSquare;
        _sumSquare = 0;
  
        if (count == lastCount)
          return 0;
  
        double avg = (sum - lastSum) / (count - lastCount);
        double part = (count - lastCount) * sumSquare - sum * sum;
  
        if (part < 0)
          part = 0;
  
        double std = Math.sqrt(part) / (count - lastCount);
  
        return _scale * (avg + n * std);
      }
    }
docs: Return the probe's next 2-sigma