The example given in the text is of the relationship between a fire alarm, smoke, and fire. 

Fire causes smoke, which causes the fire alarm to go off. The three links are necessary in the graph because fire does not directly cause the fire alarm to go off. Therefore if we know that there is smoke, the presence of fire does not lift nor lower the probability of hearing the fire alarm go off. 

University of Michigan - Ann Arbor

When the junction $$A \rightarrow B \rightarrow C$$ appears in a causal diagram, we note that the presence of $$B$$ "screens-off" the value of $$A$$. This means that $$C$$ and $$A$$ are conditionally independent given the value of $$B$$.

Chain $$A \rightarrow B \rightarrow C$$: The Screening-Off Effect

Pearl, J., & Mackenzie, D. (2018). $\mathit{The \ Book \ of \ Why: \ The \ New \ Science \ of \ Cause \ and \ Effect.}$ Basic Books.

The Book of Why

Example of the Screening Off Effect

The mediator is a variable that functions as the mechanism by way of transferring an effect of one thing to another.  

Consider the chain junction, where $A -> B -> C$.  $B$ is the mediator that transfers the effect of $A$ to $C$.   In other words, the mediator, $B$, is the variable that explains the causal effect of $A$ on $C$.  $A$ can be an indirect cause of $C$, meaning that $A$ affects $B$, which in turn contributes to the change in $C$.  

Learn Before

Related