Getting a Little Jumpy: Regression Discontinuity Designs
But when you start exercising those rules, all sorts of processes start to happen and you start to find out all sorts of stuff about people… Its just a way of thinking about a problem, which lets the shape of the problem begin to emerge. The more rules, the tinier the rules, the more arbitrary they are, the better.
Douglas Adams, Mostly Harmless (1995)
Regression discontinuity (RD) research designs exploit precise knowledge of the rules determining treatment. RD identification is based on the idea that in a highly rule-based world, some rules are arbitrary and therefore provide good experiments. RD comes in two styles, fuzzy and sharp. The sharp design can be seen as a selection-on-observables story. The fuzzy design leads to an instrumental-variables-type setup.