You’re gonna keep ‘em d-separated
In the previous chapter, we learned that colliders have a unique conditional independence pattern that sets them apart from chains and forks. The idea of -separation builds on these properties. In general, we say that two nodes in a directed acyclic graph (DAG)
are
-separated when all paths between them are blocked. When is a path between two nodes blocked?
A simple answer is when there’s a collider on a path between them or if there’s a fork or a chain that contains another variable that we control for (or a descendant of such a variable).
Let’s formalize this definition and make it a little bit more general at the same time. Instead of talking about blocking a path between two nodes with another node, we will talk about paths between sets of nodes blocked by another set of nodes. We will denote sets of nodes with capital cursive script letters, ,
, and
.
Thinking in terms of sets of nodes rather than single...