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. 2021 Feb 5;11(2):e040560. doi: 10.1136/bmjopen-2020-040560

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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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Conceptual model using directed acyclic graph (DAG) for factors associated with hospital stay. Based on current knowledge on chronic obstructive pulmonary disease (COPD) risk factors, we developed a conceptual model using DAG. Potential covariates at baseline were grouped into five blocks: patient demographic characteristics (age, gender), COPD complications, comorbidities, hospital factors (emergency admission, weekday of admission) and environmental factors (season, ozone and PM_2.5 at admission).