---- Acknowledgements ----
Please note that the second author was previously a member of the Board of Managers of Standard & Poor's Financial Services LLC and also serves as a consultant to the company. The opinions and views expressed in this article do not necessarily reflect the opinions and views of Standard & Poor's Financial Services LLC. We thank Steve LeRoy, Larry White for helpful comments. The views expressed herein are those of the authors and do not necessarily reflect the views of the National Bureau of Economic Research.