The point here is not the face masks; it’s the impossibility of assumption-free causal inference when the different treatments are entangled in this way.

Adam Pearce writers: When I read your Another Regression Discontinuity Disaster post last year, I was curious how much shifting the breakpoint would change the fit lines. A covid paper making the rounds this weekend used a similar technique so I hooked it up to an interactive widget that lets you tweak the start and …