Under this assumption it is assumed that X and Y are not common consequences of the same hidden variables ( X ← Z → Y ).

This causal sufficiency assumption is made by many methods of the cause-effect pair literature as it allows to considerably simplify the cause-effect pair problem. However there are many realistic cases where there are potential confounding effects that can affect both variables (the most typical examples of confounding factors are the age or the gender in epidemiological studies).