d-separation
variables and are said to be -separated, if all paths from to are blocked. A path may be blocked by a set of variables , if the path contains at least one arrow-emitting variable that is in or if the path contains at least one collider that is not in S and has no descendent in . A collider is defined as a variable on a path such that the path enters and leaves via arrow heads. and are, therefore, -separated conditional on , if blocks all paths between and . X and Y are marginally independent when S contains the null set