someone may ask why wiggling A makes C vary. Is it really the direct effect of A, or is it the effect of a mediating variable B? If both, can we assess what portion of the effect is mediated by B? To answer such mediation questions, we have to envision two simultaneous interventions: wiggling A and holding B constant (to be distinguished from conditioning on B). If we can perform this intervention physically, we obtain the answer to our question. But if we are at the mercy of observational studies, we need to emulate the two actions with a clever set of observations. Again, the graphical
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