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In pursuit of balance : randomization in practice in development field experiments

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  • Bruhn, Miriam
  • McKenzie, David

Abstract

Randomized experiments are increasingly used in development economics, with researchers now facing the question of not just whether to randomize, but how to do so. Pure random assignment guarantees that the treatment and control groups will have identical characteristics on average, but in any particular random allocation, the two groups will differ along some dimensions. Methods used to pursue greater balance include stratification, pair-wise matching, and re-randomization. This paper presents new evidence on the randomization methods used in existing randomized experiments, and carries out simulations in order to provide guidance for researchers. Three main results emerge. First, many researchers are not controlling for the method of randomization in their analysis. The authors show this leads to tests with incorrect size, and can result in lower power than if a pure random draw was used. Second, they find that in samples of 300 or more, the different randomization methods perform similarly in terms of achieving balance on many future outcomes of interest. However, for very persistent outcome variables and in smaller sample sizes, pair-wise matching and stratification perform best. Third, the analysis suggests that on balance the re-randomization methods common in practice are less desirable than other methods, such as matching.

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Bibliographic Info

Paper provided by The World Bank in its series Policy Research Working Paper Series with number 4752.

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Date of creation: 01 Oct 2008
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Handle: RePEc:wbk:wbrwps:4752

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Keywords: Statistical&Mathematical Sciences; Scientific Research&Science Parks; Science Education; Economic Theory&Research; Climate Change;

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  1. Banerjee, Abhijit & Cole, Shawn & Duflo, Esther & Linden, Leigh, 2006. "Remedying Education: Evidence from Two Randomized Experiments in India," CEPR Discussion Papers 5446, C.E.P.R. Discussion Papers.
  2. Khwaja, Asim Ijaz & Andrabi, Tahir & Das, Jishnu & Zajonc, Tristan, 2009. "Do Value-Added Estimates Add Value? Accounting for Learning Dynamics," Scholarly Articles 4435671, Harvard Kennedy School of Government.
  3. Duflo, Esther & Hanna, Rema & Ryan, Stephen, 2008. "Monitoring Works: Getting Teachers to Come to School," CEPR Discussion Papers 6682, C.E.P.R. Discussion Papers.
  4. Marianne Bertrand & Simeon Djankov & Rema Hanna & Sendhil Mullainathan, 2007. "Obtaining a Driver's License in India: An Experimental Approach to Studying Corruption," The Quarterly Journal of Economics, MIT Press, vol. 122(4), pages 1639-1676, November.
  5. Paul Glewwe & Albert Park & Meng Zhao, 2006. "The impact of eyeglasses on the academic performance of primary school students: Evidence from a randomized trial in rural china," Natural Field Experiments 00254, The Field Experiments Website.
  6. Edward Miguel & Michael Kremer, 2004. "Worms: Identifying Impacts on Education and Health in the Presence of Treatment Externalities," Econometrica, Econometric Society, vol. 72(1), pages 159-217, 01.
  7. Paul Glewwe & Michael Kremer & Sylvie Moulin & Eric Zitzewitz, 2000. "Retrospective vs. Prospective Analyses of School Inputs: The Case of Flip Charts in Kenya," NBER Working Papers 8018, National Bureau of Economic Research, Inc.
  8. Nava Ashraf & Dean S. Karlan & Wesley Yin, 2005. "Tying Odysseus to the Mast: Evidence from a Commitment Savings Product in the Philippines," Working Papers 917, Economic Growth Center, Yale University.
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  1. A pre-analysis plan checklist
    by David McKenzie in Development Impact on 2012-10-28 20:40:08
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