english: But even when this has been done, the very nature of epidemiological studies, which examine differences between preexisting groups, rather than deliberately inducing changes within the same individuals, means that a single study can only demonstrate a correlation between a substance and a health outcome, rather than a true cause and effect relationship.
non_english: Ali čak i kada je to obavljeno, sama priroda epidemioloških studija, koje istražuju razlike između već postojećih grupa, a ne izaziva namerne promene kod istih pojedinaca, znači da jedna studija može samo da prikaže korelaciju između supstance i zdravstvenog ishoda, a ne i pravi uzrok i posledice te veze.