 It was also called unconfoundedness, selection on the observables, or no omitted variable bias.Formally it has been written as [Yi1, Yi0] ⊥ Txi, or in words the potential Y outcome of person i had they been treated or not does not depend on whether they have really been (observable) treated or not.We can ignore in other words how people ended up in one vs. the other condition, and treat their potential outcomes as exchangeable.