…and more
In this short section, we’ll introduce and briefly discuss three assumptions: the modularity assumption, stable unit treatment value assumption (SUTVA), and the consistency assumption.
Modularity
Imagine that you’re standing on the rooftop of a tall building and you’re dropping two apples. Halfway down, there’s a net that catches one of the apples.
The net performs an intervention for one of the apples, yet the second apple remains unaffected.
That’s the essence of the modularity assumption, also known as the independent mechanisms assumption.
Speaking more formally, if we perform an intervention on a single variable 
, the structural equation for this variable will be changed (for example, set to a constant), yet all other structural equations in our system of interest will remain untouched.
Modularity assumption is central to do-calculus as it’s at the core of the logic of interventions.
Let’s see...
