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. 2019 Jan 8;19:8. doi: 10.1186/s12886-018-1016-7

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© The Author(s). 2019

Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided 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 Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.

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Fig. 2 — Assessment of the risk of bias in the included studies. a: Risk of bias summary: the authors’ judgements on each risk of bias item for the included studies. +: low risk of bias; −: high risk of bias; and?: unclear risk of bias. b: Risk of bias graph: authors’ judgements regarding each risk of bias item displayed as percentages across the included studies