Discussion on "Instrumented difference-in-differences" by Ting Ye, Ashkan Ertefaie, James Flory, Sean Hennessy, and Dylan S. Small.

We discuss Ye et al. 2022, which combines instrumental variables methods with difference in differences. First, we compare the paper to other works in the difference in differences literatures and argue that the main contribution lies in the multiply robust estimation approach. Then, we reformulate the causal assumptions in Ye et al. 2022 in the usual theoretical framework of the instrumental variables literature. This clarifies in which sense the difference in differences design can weaken the standard instrumental variable conditions.
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