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The impact of exacerbation frequency on mortality following acute exacerbations of COPD: a registry-based cohort study
Sigrun Alba Johannesdottir Schmidt, Martin Berg Johansen, Morten Olsen, Xiao Xu, Joseph M Parker, Nestor A Molfino, Timothy L Lash, Henrik Toft Sørensen, Christian Fynbo Christiansen

BMJ Open Dec 2014, 4 (12) e006720; DOI: 10.1136/bmjopen-2014-006720

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Time-varying exposure