* Double#NEGATIVE_INFINITY}, or {@link Double#NaN}) then the result is {@link Double#NaN}.
   *
   * @throws IllegalStateException if the dataset is empty
   */
  public final double populationVariance() {
    checkState(count != 0);
    if (isNaN(sumOfSquaresOfDeltas)) {
      return NaN;
    }
    if (count == 1) {
      return 0.0;
    }