## Causal Arrow

A causal arrow is a link from a chance node (discrete or continuous) or a decision node to a chance node.

It defines a causal dependency from the parent node to the child node. That is, the state of the parent has impact on the state of the child.

It can sometimes be difficult to determine what is a causal dependency and what is not. Eg. imagine you have two nodes: One for "Red Spots" and one for "Measles". Some might say that when you observe red spots on a child it is very likely that the child has measles so you should add a link from "Red Spots" to "Measles". This, however, is terribly WRONG! The relation between red spots and measles is that measles causes red spots. The link should go from "Measles" to "Red Spots" (because it is a causal arrow).

Always make sure that the causal arrows you add are causal.

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