is marker for is marker for C is marker for d if and only if the presence or occurrence of d is correlated with the presence of occurrence of c, and the observation of c is used to infer the presence or occurrence of d. Note that this does not imply that c and d are in a direct causal relationship, as it may be the case that there is a third entity e that stands in a direct causal relationship with c and d. [RO] correlated with