NBER Working Paper No. 18499
---- Acknowledgements -----
We gratefully acknowledge the financial support of the National Science Foundation through its SciSIP Program (Awards SBE-0738142, SBE-0965364, and SBE-0738394). We thank Heidi Williams and participants in the MIT Labor Lunch and NBER Productivity Lunch for insightful comments. Vivienne Groves and Mikka Rokkanen provided additional research assistance. The project would not have been possible without Andrew Stellman's extraordinary programming skills (http://www.stellman-greene.com/). The usual disclaimer applies. The views expressed herein are those of the authors and do not necessarily reflect the views of the National Bureau of Economic Research.