* Double#NEGATIVE_INFINITY}, or {@link Double#NaN}) then the result is {@link Double#NaN}.
   *
   * @throws IllegalStateException if the dataset is empty or contains a single value
   */
  public double sampleVariance() {
    checkState(count > 1);
    if (isNaN(sumOfSquaresOfDeltas)) {
      return NaN;
    }
    return ensureNonNegative(sumOfSquaresOfDeltas) / (count - 1);
  }

  /**
   * Returns the <a