Step (2) of Pedro's DiD checklist: documenting how many units are in each cohort

Happy Father's Day!!

I am going to continue walking us through Pedro Sant’Anna’s difference-in-differences checklist (“Pedros checklist”) with a focus on step 2. It’s pretty straightforward, but still as I wanted to show code, I thought I’d make it just one substack entry rather than combine it with step 3. Step 2 is “Document how many units are treated in each cohort.”

Fo…