<prompt> The most difficult criterion to establish Causal association in etiology of a disease is-TemporalityStrength of associationSpecificity of associationBiological plausibility  <response>: 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 Hills criteria of causal association Surgeon generals criteria include: Temporality Consistency Strength Biological plausibility Coherence Dose response relationship Cessation of exposure. 