More complex dose–response relationships may exist, and modern studies have confirmed that a monotonic dose–response curve is an overly simplistic representation of most causal relationships.


University of Michigan - Ann Arbor

Hill wrote that “if a dose response is seen, it is more likely that the association is causal.” According to the traditional interpretation of biological gradient, a monotonic biological gradient, wherein increased exposure resulted in increased incidence of disease, provides the clearest evidence of a causal relationship. 


Criteria 6: Biological gradient

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4589117/

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