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Geographic Boundaries as Regression Discontinuities

Published online by Cambridge University Press:  04 January 2017

Luke J. Keele
Affiliation:
Department of Political Science, 211 Pond Lab, Penn State University, University Park, PA 16802, e-mail: ljk20@psu.edu
Rocío Titiunik*
Affiliation:
Department of Political Science, University of Michigan, 5700 Haven Hall, 505 South State Street, Ann Arbor, MI 48109-1045
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Abstract

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Political scientists often turn to natural experiments to draw causal inferences with observational data. Recently, the regression discontinuity design (RD) has become a popular type of natural experiment due to its relatively weak assumptions. We study a special type of regression discontinuity design where the discontinuity in treatment assignment is geographic. In this design, which we call the Geographic Regression Discontinuity (GRD) design, a geographic or administrative boundary splits units into treated and control areas, and analysts make the case that the division into treated and control areas occurs in an as-if random fashion. We show how this design is equivalent to a standard RD with two running variables, but we also clarify several methodological differences that arise in geographical contexts. We also offer a method for estimation of geographically located treatment effects that can also be used to validate the identification assumptions using observable pretreatment characteristics. We illustrate our methodological framework with a re-examination of the effects of political advertisements on voter turnout during a presidential campaign, exploiting the exogenous variation in the volume of presidential ads that is created by media market boundaries.

Information

Type
Research Article
Copyright
Copyright © The Author 2014. Published by Oxford University Press on behalf of the Society for Political Methodology