met, the identified mean E(y|δ=1) is not equal to the desired mean E(y). But the monotonicity condition E(y|δ=1)â … monotonicity. The main question is then whether the monotonicity condition is testable, and if not, when it is plausible. Answering … monotonicity; when z is not available, we provide primitive conditions and plausible models for the monotonicity. Going further …