The most difficult criterion to establish Causal association in etiology of a disease is-
Specificity of association means that the disease is caused by a paicular risk factor. It is difficult criterion to establish, as it is difficult to rule out the same outcomes by other exposures. Other Hill's criteria of causal association/ Surgeon general's criteria include: Temporality Consistency Strength Biological plausibility Coherence Dose response relationship Cessation of exposure.
Temporality
Strength of association
Specificity of association
Biological plausibility
Epidemiology