What is the primary type of evidence demonstrated by observational studies when examining two variables?

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Observational studies reveal how two variables relate in real-world settings, showing an association. Because there is no random assignment or deliberate manipulation, these studies can show that the variables move together, but not that one causes the other. The link could be due to confounding factors, reverse causation, or bias. To claim causation, you’d need a design with manipulation and randomization to control for confounders. Therefore, the primary type of evidence these studies provide is an association between the variables.