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. 2012 Jun 29;7(6):e39272. doi: 10.1371/journal.pone.0039272

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Zhang et al.

This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are properly credited.

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The reasons for exclusion included: no relevant outcome reported (n = 895); duplicates (n = 97); not designed to discuss the key question (n = 77); not considered an original publication or research (i.e., letters, abstracts, reviews, editorials, etc.) (n = 6); poor quality (n = 10, including studies containing data on patients that were not in accordance with our diagnostic criteria (n = 9); or the study was not designed as a cohort study (n = 1)). Therefore, two prospective cohort studies and nine retrospective cohort studies were included in our meta-analysis.