What can we do?
SWIG stands for Single World Intervention Graph. These graphs help us visualize the interventions we want to do and the potential consequences of those interventions.
And because when we use SWIGs we can see that many of the common mediation questions people ask require us to know about multiple possible worlds all at once.
•controlled direct effect
•natural direct effect
In practice, the SWIG will help us see that we can really only ever estimate controlled direct effects!
The expected change in blood pressure when everyone gets 40mg Drug X vs 0mg Drug X *and* exercise is fixed to some set value, say 30 minutes.
Although we often don’t think about it, “fixing” exercise implies an intervention.
That’s because there are infinitely many controlled direct effects where exercise was set to some value. The direct effect of the drug might differ depending on which value we pick.
What about the natural direct effect?
¹Unless we want to make some pretty strong assumptions & personally, I don’t!
If you want to know how much Drug X changes blood pressure, separate from changes in exercise, you have to tell us what level(s) of exercise you care about!
There’s no such thing as THE direct effect!
For fun, let’s end with a poll. How do you feel right now: