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. 2017 Nov 20;7(11):e015640. doi: 10.1136/bmjopen-2016-015640

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This is an Open Access article distributed in accordance with the terms of the Creative Commons Attribution (CC BY 4.0) license, which permits others to distribute, remix, adapt and build upon this work, for commercial use, provided the original work is properly cited. See: http://creativecommons.org/licenses/by/4.0/

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Six causal diagrams were designed for estimating the causal effect of E on D. (A) a single mediator M; (B) two series mediators M₁ and M₂; (C) two independent parallel mediators M₁ and M₂; (D) two correlated parallel mediators M₁ and M₂; (E) a single mediator with an unobserved confounder U; (F) two independent parallel mediators M₁ and M₂ with an unobserved confounder U.