Statistical association

A statistical association is a systematic dependence between variables. It need not represent a causal effect, because it may arise from confounding, selection, or reverse causation.

An observational study measures exposures and outcomes without assigning the exposure by an intervention. Its causal interpretation therefore requires assumptions that address confounding and selection bias.

Reverse causality occurs when an apparent exposure-outcome association is partly or wholly generated by the outcome, or by an early stage of it, affecting the measured exposure.