code: public double populationVariance() {
      checkState(count > 0);
      if (isNaN(sumOfSquaresOfDeltas)) {
        return NaN;
      }
      if (count == 1) {
        return 0.0;
      }
      return ensureNonNegative(sumOfSquaresOfDeltas) / count();
    }
docs: 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
  is not guaranteed to return zero when the dataset consists of the same value multiple times,
  due to numerical errors. However, it is guaranteed never to return a negative result.
  
  <h3>Non-finite values</h3>
  
  <p>If the dataset contains any non-finite values ({@link Double#POSITIVE_INFINITY}, {@link
  Double#NEGATIVE_INFINITY}, or {@link Double#NaN}) then the result is {@link Double#NaN}.
  
  @throws IllegalStateException if the dataset is empty