I am conducting a mediation analysis with the mediation package in R. As a result I get values for an average direct effect (ADE) and an average causal mediation effect (ACME).

In the literature such mediations are often drawn as path models in which X represents the predictor, M the mediator and Y the dependent variable. X -> Y is then called c', X -> M becomes a and M -> Y is called b.

As I see it, the ADE represents the path c'. ACME is the product of paths a and b, that is ab.

How do I get values for a and b?

Your help is much appreciated!


1 Answer 1


When you run mediate(), you need to supply two models: the mediator model (M ~ T + X1 + X2) and the outcome model (Y ~ M + T + X1 + X2). The coefficient for T in the mediator model is a, and the coefficient for M in the outcome model is b. Note that these values only (directly) relate to the ACME and ADE if the outcome and mediator are continuous and there is no treatment-mediator interaction on the outcome.


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