A causal path is a route through a directed acyclic graph from X to Y whose arrows consistently point forward: X → Z1 → … → Y. It represents one way changing X can causally change Y. A path such as X ← Z → Y is not causal because changing X does not change Z along that route. section 6.1.2 section 6.4
Topic, platform, and pageviews
Suppose topic affects pageviews directly, or affects the platform that then affects pageviews. The platform is a mediator when estimating topic's effect on pageviews, but becomes the treatment when comparing platforms. These roles determine which causal paths and effects matter. section 6.1.2
Correlation can travel through both causal and noncausal paths. The pipe-network analogy treats variables as nodes and correlation as gas: conditioning on a confounder can close a path such as X ← Z → Y, leaving the causal route X → Y. In a graph, confounders, mediators, and colliders therefore require different adjustment decisions. section 6.2.1
Use path structure to interpret effects and linear calculations:
- Trace only forward-directed paths when identifying causal effects from X to Y. section 6.1.2
- For standardized linear graphs, multiply coefficients along each relevant path; total effects combine directed causal paths, while correlations may also include noncausal paths without colliders. section 6.2.3
- Choose regression controls only after distinguishing confounders, mediators, and colliders and deciding whether the target is a direct, indirect, or total effect. section 6.4
