@Override
  public String toString() {
    if (count() > 0) {
      return MoreObjects.toStringHelper(this)
          .add("count", count)
          .add("mean", mean)
          .add("populationStandardDeviation", populationStandardDeviation())
          .add("min", min)
          .add("max", max)
          .toString();
    } else {
      return MoreObjects.toStringHelper(this).add("count", count).toString();
    }
  }

   * Double#NEGATIVE_INFINITY}, or {@link Double#NaN}) then the result is {@link Double#NaN}.
   *
   * @throws IllegalStateException if the dataset is empty
   */
  public final double populationStandardDeviation() {
    return Math.sqrt(populationVariance());
  }

  /**
   * Returns the <a href="http://en.wikipedia.org/wiki/Variance#Sample_variance">unbiased sample