 Later, in 1983  Rubin and Rosenbaum rather define "strongly ignorable treatment assignment" which is a stronger condition, mathematically formulated as formula_1, where formula_2 is a potential outcome given treatment formula_3, formula_4 is some covariates and formula_5 is the actual treatment.Pearl [2000] devised a simple graphical criterion, called "back-door", that entails ignorability and identifies sets of covariates that achieve this condition.Ignorability (better called exogeneity) simply means we can ignore how one ended up in one vs. the other group (‘treated’ Tx = 1, or ‘control’ Tx = 0) when it comes to the potential outcome (say Y).