Unmeasured confounding and selection into treatment are key threats to reliable causal inference in Difference-in-Differences (DiD) designs. In practice, researchers often use instrumental variables to address endogeneity concerns, for example through shift-share instruments. However, in many settings instruments may be correlated with unobserved confounders, exhibiting pre-trends. In this paper we explore the use of synthetic controls to address unmeasured confounding in IV-DiD settings. We propose a synthetic IV estimator that partials out the unmeasured confounding and derive conditions under which it is consistent and asymptotically normal, when the standard two-stage least squares is not. Motivated by the finite sample properties of our estimator we then propose an ensemble estimator that might address different sources of bias simultaneously. We illustrate our method through a simulation exercise and two shift-share empirical applications: the Syrian refugee crisis effect on Turkish labor markets and the impact of Chinese imports on US manufacturing employment.
Synthetic instruments in DiD designs with unmeasured confounding
jaume vives-i-bastida,Ahmet Gulek
Published 2024 in Social Science Research Network
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2024
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Social Science Research Network
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