Use of Causal Diagrams to Inform the Design and Interpretation of Observational Studies: An Example from the Study of Heart and Renal Protection (SHARP)

Table 2.

Rules to establish whether a pathway in a causal diagram is open or blocked

Rule No.	Type of Path	No. of Colliders on Path	Aim of Adjustment	How to Establish if Path is Open or Blocked
1	Causal	N/A	Leave open (for estimation of total causal effect)	The path will be open providing that no variables along it are conditioned on (otherwise, it will be blocked)
2	Noncausal	0	Block noncausal pathway	The path will be blocked if at least one variable along it is conditioned on (otherwise, it will be open)
3a	Noncausal	1	Block noncausal pathway	The path will be open if the only variable conditioned on is the collider^a (otherwise, it will be blocked)
3b	Noncausal	>1	Block noncausal pathway	The path will be open if all of the collider variables^a (and no noncollider variables) are conditioned on (otherwise, it will be blocked)

N/A, not applicable.

^aOr descendant(s) of the collider(s).