To measure the average total effect of $X$ on $Y$, we need to measure the following two: 1. We can estimate the average causal effect (ACE) of a unit increase in $X$ on $M$ because the back-door path X leftarrow C rightarrow Y leftarrow M is blocked by the collider at $Y$. ACE_{X rightarrow M} = P(M|X) - P(M|no X) 2. To measure the ACE of M rightarrow Y, we can block the back-door path M leftarrow X leftarrow C rightarrow Y by controlling for $X$. ACE_{M rightarrow Y} = P(Y|do(M)) - P(Y|do(no M)) 3. According to the back-door adjustment formula: $P(A|do(B)) = \sum_{C} P(A|B, C=c)P(C=c)$ Rightarrow P(Y|do(X)) = sum_{M} P(M=m|X) sum_{X} P(Y|X=x,M=m)P(X=x) Rightarrow ACE_{X rightarrow Y} = ACE_{X rightarrow M} * ACE_{M rightarrow Y}