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. 2016 May 20;125(1):23–29. doi: 10.1289/EHP232

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Directed acyclic graph for the Granger causality model. Confounder U2 is measured and controlled, but confounder U1 is not. POL_b is pollution before the outcome (O), and POL_a is pollution after the outcome. If U1 is not controlled, there is a backdoor path from O to POL_a, and an association would be expected. Hence, failure to find an association is evidence of a lack of confounding (i.e., no U1).