April 13, 2021
Breaks \(W \rightarrow X\) link
|All confounding variables||1. \(X\) is random
2. Change only \(X\)
|1. Condition on all confounders
2. Low measurement error
|Before and After||Hold confounders
change w/ \(X\)
|Diff in Diff||Hold confounders
|Parallel trends /
Expectation is you can answer:
Mueller and Schwarz (2020) investigate:
Did Trump’s tweeting of anti-Muslim messages increase anti-Muslim hate crimes?
We can’t observe the US in the absence of Trump tweeting against Muslims, so authors use correlation…
Trump’s Twitter gained attention as he ran for President.
Trump made nearly 300 negative tweets about Muslims.
When Trump gained prominence, anti-Muslim hate crimes increased
Even comparing US to itself over time…
Counties with more SXSW Twitter Joiners (treated) see larger increase in hate crimes following rise of Trump’s Twitter
Days with Trump golfing followed by more hate crimes
With reasonable assumptions (no different trends in places with more SXSW 2007 attendees on days when Trump golfs), social media rhetoric causes hate crimes.
Karaivanov et al (2020), economists at SFU, investigate:
Have indoor mask mandates reduced COVID cases, on average?
Could be that mask mandates…
Karaivanov et al use a variation on Difference in Difference…
But if mask mandate places
Karaivanov et al address confounding by variables that change over time differently…
Estimate counterfactual world…