Potential Outcomes model of Causality

Don Green

How would an experimental subject have responded if treated?
How would same subject have responded if untreated?
Difference between these two potential outcomes is the unit-level treatment effect
Average unit-level treatment effect is the ATE (average treatment effect in the subject pool)
average treatment effect is the target estimate

So the two ways to think about, then, the average treatment effect is it's the average of the unit level treatment effects. Or equivalently, it's the extent to which the world changes if the entire subject pool goes from untreated to treated.