Random Thoughts

Lauren Jones from U Toronto asks

I have a question about semantics.  You use the word "random" to
describe both the treatment assignment variables D in RCTs, and also
to describe the outcome variables Y in regression analysis. "Random"
means something different in the first application than in the second.
Indeed, you explain that Y varies with "systematic randomness" (MHE p.
29), but I am still a little confused about how to square away the two
uses of the word. Any help?

Good question, lauren.  There are many sorts of randomness in the 
'Metrics world: random variables, random samples, randomized trials. 
We try to make the distinction clear in our new book, 
Mastering 'Metrics