In observational studies, the relationship between two variables is typically described as:

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In observational research, a link between two variables is described as an association. Researchers observe what exists and note how the variables tend to vary together, but they do not assign exposure or intervention to participants. Because there is no randomization to control for confounding factors, an observed association might be influenced by other variables that affect both sides of the relationship. This is why we stop at association rather than claiming causation.

Randomization is a design feature used in experiments to balance unknown factors between groups, not a description of the relationship itself. Control refers to strategies to adjust for confounding or to manage study conditions, but it does not define the intrinsic relationship being measured. So, the best way to characterize what’s observed in observational studies is an association.