In social science research, interference between units is the rule
rather than the exception. 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.
In this paper, we consider the identification and sensitivity
analysis of contagion effects. 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
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 a non-contacted voter by
altering the vote intention of a contacted voter or through other
mechanisms. To address the potential violation of key
identification assumptions, we propose nonparametric and parametric
sensitivity analyses. We find robust contagion effects among some
households. |