@Override
  public String toString() {
    if (count() > 0) {
      return MoreObjects.toStringHelper(this)
          .add("xStats", xStats)
          .add("yStats", yStats)
          .add("populationCovariance", populationCovariance())
          .toString();
    } else {
      return MoreObjects.toStringHelper(this)
          .add("xStats", xStats)
          .add("yStats", yStats)
          .toString();
    }
  }

   * product-moment correlation coefficient</a> of the values. The count must greater than one, and
   * the {@code x} and {@code y} values must both have non-zero population variance (i.e. {@code
   * xStats().populationVariance() > 0.0 && yStats().populationVariance() > 0.0}). The result is not
   * guaranteed to be exactly +/-1 even when the data are perfectly (anti-)correlated, due to
   * numerical errors. However, it is guaranteed to be in the inclusive range [-1, +1].

   */
  public final double sum() {
    return mean * count;
  }

  /**
   * Returns the <a href="http://en.wikipedia.org/wiki/Variance#Population_variance">population
   * variance</a> of the values. The count must be non-zero.
   *
   * <p>This is guaranteed to return zero if the dataset contains only exactly one finite value. It

   */
  public double sum() {
    return mean * count;
  }

  /**
   * Returns the <a href="http://en.wikipedia.org/wiki/Variance#Population_variance">population
   * variance</a> of the values. The count must be non-zero.
   *
   * <p>This is guaranteed to return zero if the dataset contains only exactly one finite value. It