External Validity and Partner Selection Bias
---- Acknowledgements -----
We thank Josh Angrist, Amitabh Chandra, Lucas Davis, Angus Deaton, Meredith Fowlie, Xavier Gine, Chuck Goldman, Matt Harding, Joe Hotz, Guido Imbens, Larry Katz, Dan Levy, Jens Ludwig, Konrad Menzel, Rohini Pande, Todd Rogers, Ed Vytlacil, and seminar participants at the ASSA meetings, Berkeley, Harvard, MIT, NBER, NEUDC, the UCSB/UCLA Conference on Field Experiments, and the World Bank for insights and helpful comments. Thanks also to Tyler Curtis, Marc Laitin, Alex Laskey, Dan Yates, and others at Opower for fruitful discussions. Piyush Tantia and Scott Gaul provided useful help in accessing the Microfinance Information Exchange data. We are grateful to the Sloan Foundation for financial support for this paper and our related research on the economics of energy efficiency. The views expressed herein are those of the authors and do not necessarily reflect the views of the National Bureau of Economic Research.