Illustration of Assumption 3 and its distinction compared to the monotonicity assumption using seven hypothetical units. Units and are allowed under both assumptions, Assumption 3 and monotonicity. For unit , APOE expedites both AD and death, but in a way that AD remains before death. For unit 2, APOE impedes AD but expedites death. Nevertheless, unit 2 would live long enough under APOE for AD to occur. Similar protective effect occurs for unit , however, this unit violates monotonicity at time (as an example). Unit also violates monotonicity (at and ) but not Assumption 3. Unit does not suffer from AD under any APOE value, hence it is allowed by Assumption 3 but it does violate monotonicity (at and ) as AD carries a protective effect against death. Unit violates Assumption 3 but not monotonicity. Arguably, for this unit the APOE effect on death is so strong the unit would have died under APOE before getting the chance to have AD. Finally, unit violates both assumptions.