Concepts de base
Common cause principle resolves Simpson's paradox by considering unobserved variables.
Stats
"If B and C are binary and A is quaternary, the conditioning over any binary common cause C establishes the same direction of the association between a1 and a2 as the conditioning over B in the original formulation of the paradox."
"For tertiary (unobserved) common causes C all three options of Simpson’s paradox become possible (i.e. marginalized, conditional, and none of them), and one needs prior information on C to choose the right option."
Citations
"Simpson’s paradox is an obstacle to establishing a probabilistic association between two events a1 and a2, given the third (lurking) random variable B."
"Common causes that are close to B imply option (2, 3) of the paradox, while C ≈ A leads to (1)."