Fig 3. Diagram illustrating the rank preserving assumption for a dichotomous instrumental variable Z ∈ {0, 1} with counterfactual exposure distributions X(0) (the black group) and X(1) (the blue group).

The dashed arrow represents the one-to-one mapping from the counterfactual exposure value with Z = 0 to the counterfactual covariate value with Z = 1.