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This analysis is really frustrating, and not something I would signed off on at @FiveThirtyEight. Let me try and go through some of the reasons why. https://www.nytimes.com/2020/04/21/us/politics/wisconsin-mail-voting.html
1. The framing: one election doesn't "upend" a whole host of much better evidence showing that vote-by-mail tends to have neutral effects, most recently e.g. http://www.andrewbenjaminhall.com/Thompson_et_al_VBM.pdf by @danmthomp and others.
*Especially* an election held under such unusual circumstances.
*Especially* an election held under such unusual circumstances.
2. The article says that none of the prior studies have compared in-person and mail results. But that's not actually interesting, since people who vote by mail differ from those who vote in person. That's especially true in this case...
since we know that how seriously people take COVID-19 differs by party (e.g. https://www.vox.com/science-and-health/2020/3/31/21199271/coronavirus-in-us-trump-republicans-democrats-survey-epistemic-crisis). So showing that mail votes are more Democratic in an environment in which staying home is partisan doesn't tell you anything about what more widespread VBM would do.
The parties also had different strategies: Democrats called for their voters to vote by mail (see e.g. https://www.politico.com/news/2020/04/01/wisconsin-dems-support-postponing-spring-elections-160318), while Republicans didn't. It would be weird if we *didn't* see a difference in how the different parties voted.
And Democrats and Republicans live in different areas (even within municipalities, see e.g. https://projects.fivethirtyeight.com/republicans-democrats-cities/), so even if they did take COVID similarly seriously, Democrats might be likelier to want to vote by mail than Republicans.
3. The two parties had different incentives to turn out. (I don't want to overplay this, since the primary wasn't that competitive anymore. But the composition of the electorate still differs from a general election in which both parties have an incentive to turn out.)
