An observational study finds that commuters who bike to work have lower average blood pressure. State the association and assess direct benefit, healthier-person selection, and walkable activity-friendly neighborhoods as plausible explanations.
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A two-variable association can be compatible with several causal diagrams. Test a direct pathway, a reverse-selection pathway, and a common cause separately, asking whether each could produce the same observed pattern; without assignment or stronger evidence, the data do not identify which diagram generated it.
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