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. 2019 Sep 18;10:973. doi: 10.3389/fphar.2019.00973

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Copyright © 2019 Ali, Prieto-Alhambra, Lopes, Ramos, Bispo, Ichihara, Pescarini, Williamson, Fiaccone, Barreto and Smeeth

This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

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Causal diagrams representing time-varying treatment (AZT), outcome (progression to AIDS, AIDS), and time-varying confounding (CD4 count). Time-varying confounding is not affected by prior treatment (A), time-varying confounding is affected by prior treatment (B), time-varying confounding affected by unmeasured factor U, which is also associated with the outcome (C), and conditioning or stratifying on time-varying confounder, indicated by box around CD41), creates association between time-varying confounder CD4₀) and unmeasured factor U (D).