A Special Case of Difference-in-Differences I remember reading a paper where the Difference-in-Differences was a bit special, but I had forgotten the details. My recollection was

Two time periods, specifically one time period before treatment and one time period after treatment.
The &quot;treatment&quot; was such that all firms had to comply with a certain share of a variable X.
The treatment group consisted of firms with very low (or zero) share of X before the treatment, and
The control group consisted of firms with a share already close to the required share of X before the treatment.

The idea was that the treatment group received most of the shock, while the control group was little affected.
Do you know a paper that uses such a framework?
Take a look at Ch 5 of Mastering Metrics (Angrist and Pischke) or their earlier &quot;Mostly harmless econometrics&quot;.