---- Acknowledgements ----
This research was conducted with restricted access to Bureau of Labor Statistics data. The views expressed here are those of the authors and do not reflect necessarily the views of the BLS or the National Bureau of Economic Research. This research is supported by NSF grant #SES0617256. We owe a huge debt of gratitude to our project coordinator Rozi Ulics for the numerous hours she has spent helping us on this project. We thank Bill Alterman, Andrew Cohen, Dave Mead, Tien Nguyen, Molly Shannon and Daryl Slusher for all their invaluable help and conversations at the BLS. We owe special thanks to our editor Roberto Barro and three anonymous referees for their numerous insightful suggestions. We thank Philippe Aghion, Mark Aguiar, Pol Antras, Mark Bils, Ariel Burstein, Doireann Fitzgerald, Jordi Gali, Emi Nakamura, Ken Rogo, Julio Rotemberg, Aleh Tsyvinski and Jon Steinsson for useful comments. We also thank Oleg Itskhoki and Tim Schwuchow for excellent research assistance.