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. 2021 Jul 12;7(2):e001654. doi: 10.1136/rmdopen-2021-001654

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© Author(s) (or their employer(s)) 2021. Re-use permitted under CC BY-NC. No commercial re-use. See rights and permissions. Published by BMJ.

This is an open access article distributed in accordance with the Creative Commons Attribution Non Commercial (CC BY-NC 4.0) license, which permits others to distribute, remix, adapt, build upon this work non-commercially, and license their derivative works on different terms, provided the original work is properly cited, appropriate credit is given, any changes made indicated, and the use is non-commercial. See: http://creativecommons.org/licenses/by-nc/4.0/.

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Directed acyclic graphs (DAGs) representing (A) a causal effect in a randomised controlled trial, (B) an association in an observational study and (2C) an average marginal effect in an observational study. Black arrow, association of interest (‘front door’); red arrows, biasing paths (‘back doors’); crossed red arrows, closed biasing paths (eg, by randomisation in A and adjustment in C). T_b, treatment status at baseline (yes/no); C_b, a set of observed pretreatment confounders at baseline; Ub, a set of unobserved pretreatment confounders at baseline; Y_f, outcome at follow-up. The notation below each DAG indicates either a mean difference or a risk ratio as measures of the treatment effect. The average marginal effect is the equivalent of a causal effect of a randomised controlled trial under the assumption that all confounders had been measured and adjusted for (no unmeasured confounding).