Imai, Kosuke and Zhichao Jiang. ``Identification and Sensitivity Analysis of Contagion Effects with Randomized Placebo-Controlled Trials.''



In social science research, interference among units is the rule rather than the exception because people often interact with and influence one another. Contagion represents one key causal mechanism of such spillover effects, where one's treatment affects the outcome of another individual indirectly by changing the treated unit's own outcome. Alternatively, the treatment of one individual can affect the outcome of another person through other mechanisms without altering the treated individual's outcome. In this paper, we consider the identification and sensitivity analysis of contagion effects. In particular, we analyze a randomized placebo-controlled trial of get-out-the-vote campaign, in which canvassers were sent to randomly selected households with two registered voters but encouraged only one voter within each household to turn out in an upcoming election. To address the problem of noncompliance, the experiment includes a placebo arm, in addition to the standard treatment and control arms, in which canvassers encourage voters to recycle. We show how to identify and estimate the average contagion and direct effects by decomposing the average spillover effect. Our analysis examines whether canvassing increases the turnout of unreachable voter by altering the vote intention of reachable voter or through other mechanisms such as heightening the salience of the election in conversations among voters of the same household. To address potential violation of key identification assumptions, we propose nonparametric and parametric sensitivity analyses. We find robust contagion effects among some households. (Last Revised, October 2017)

© Kosuke Imai
 Last modified: Tue Oct 3 18:50:03 EDT 2017