The back-door criterion
The back-door criterion is most likely the best-known technique to find causal estimands given a graph. And the best part is that you already know it!
In this section, we’re going to learn how the back-door criterion works. We’ll study its logic and learn about its limitations. This knowledge will allow us to find good causal estimands in a broad class of cases. Let’s start!
What is the back-door criterion?
The back-door criterion aims at blocking spurious paths between our treatment and outcome nodes. At the same time, we want to make sure that we leave all directed paths unaltered and are careful not to create new spurious paths.
Formally speaking, a set of variables, , satisfies the back-door criterion, given a graph , and a pair of variables, if no node in is a descendant of , and blocks all the paths between and that contain an arrow into (Pearl, Glymour, and Jewell, 2016).
In the preceding definition, means that...