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. 2017 Aug 29;7:9813. doi: 10.1038/s41598-017-10230-3

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© The Author(s) 2017

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.

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Funnel plot. Note: In this figure, a spot represents an included study. With the included studies, a regression line was drawn. It could be found that the spots surrounded the regression line did not present with a good symmetry. Therefore, the existence of potential publication bias was detected in our studys.