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. 2014 Apr 8;19(1):18. doi: 10.1186/2047-783X-19-18

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Copyright © 2014 Yang et al.; licensee BioMed Central Ltd.

This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly credited.

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Survival by year in COPD patient group 1, 2A and 2B, experiencing infrequent, mildly frequent and severely frequent exacerbations respectively. Note: P_{group 1 – group 2} = 0.026, P_{group 1 – group 2B} = 0.009, P_{group 2 A– group 2B} = 0.561.