code: public double getStandardDeviation() {
          Lock lock = this.getLock().readLock();
          lock.lock();
          try {
              return this.sigma;
          } finally {
              lock.unlock();
          }
      }
docs: Return the standard deviation. The standard deviation is a measure of the variation in a series of values. Values with a
  lower standard deviation has less variance in the values than a series of values with a higher standard deviation.
  @return the standard deviation, or 0.0 if the {@link #getCount() count} is 0 or if all of the values are the same.