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UP NEXT: 2nd seed Manatee (Trichechus manatus) versus 12th seed Ring-tailed Vontsira (Galidia elegans) #2019MMM
The West Indian Manatee is a fully aquatic mammal (it never comes out of the water, like whales) with two subspecies that are found along the coast of Florida, islands of the Caribbean, & northeastern coast of South America (Husar, 1978) #2019MMM
Truly a sea cow, manatees are herbivores & graze on all sorts of water plants. In fact, one free-ranging manatee spent on average 11 hours per day feeding (Etheridge et al, 1985) #WayToGoGus #WhereIsPickle #AwesomeOldPapers #2019MMM
Read 17 tweets
Do you use diff-in-diff? Then this thread is for you.

You’re no dummy. You already know diverging trends in the pre-period can bias your results.

But I’m here to tell you about a TOTALLY DIFFERENT, SUPER SNEAKY kind of bias.

Friends, let’s talk regression to the mean. (1/N)
Flashback to 2015. @ryan_dydx @JFBurgessJr & @jdimick1 publish a simulation paper
dx.doi.org/10.1111/1475-6… (2/N)
In a scenario that should be FINE for diff-in-diff, they get CRAZY HIGH Type I error rates.

After matching on pre-period variables (via propensity scores), things do indeed look fine. (3/N)
Read 18 tweets

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