Dear Applied Statistics Workshop Community,
Our next meeting will be on Wednesday, March 20 (12:00 EST). Anton
Strezhnev presents "A Guide to Dynamic Difference-in-Differences
Regressions for Political Scientists."
<When>
March 20, 12:00 to 1:30 PM, EST
Lunch will be available for pick-up inside CGIS K354.
<Where>
In-person: CGIS K354
Zoom:
https://harvard.zoom.us/j/93217566507?pwd=elBwYjRJcWhlVE5teE1VNDZoUXdjQT09
<Abstract>
Difference-in-differences (DiD) designs for estimating causal effects have
grown in popularity throughout political science. Many DiD papers present
their central results through an "event study" plot - a visualization that
combines estimated dynamic average treatment effects for multiple
post-treatment time periods alongside placebo tests of the main identifying
assumption: parallel trends. Despite their ubiquity, the methods used in
practice for the creation of these plots are not standardized and in many
cases the approaches adopted by researchers can result in misleading
inferences about both the treatment effects and the placebo tests. Building
on and synthesizing recent contributions in the econometric literature on
differences-in-differences designs, this paper illustrates some common
pitfalls through a replication of three recently published papers in major
political science journals. We identify three notable problems related to
the incorrect specification of the baseline comparison time, incorrect
inclusion of "always-treated" units, and sensitivity to effect homogeneity
assumptions. We help provide researchers with additional intuition for the
problems that arise due to effect heterogeneity and for the "contamination
bias" result of Sun and Abraham (2021) through a novel decomposition of the
dynamic event study regression in the style of Goodman-Bacon (2021) that
allows researchers to recover the weights placed on each 2x2 comparison
used to construct the effect estimates and placebos. These weights allow
researchers to gauge the sensitivity of each estimate to potential effect
heterogeneity.
Anton is happy to meet with students and faculty after the talk. Please
reach out to Jialu directly if you want to schedule 1:1 meetings with him.
<2023-2024 Schedule>
GOV 3009 Website:
https://projects.iq.harvard.edu/applied.stats.workshop-gov3009
Calendar:
https://calendar.google.com/calendar/u/0?cid=Y18zdjkzcGF2OWZqa2tsZHJidTlzbm…
Best,
Jialu
--
Jialu Li
Department of Government
Harvard University
*https://jialul.github.io/ <https://jialul.github.io/>*