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. 2015 Jan 15;5(1):e006524. doi: 10.1136/bmjopen-2014-006524

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Published by the BMJ Publishing Group Limited. For permission to use (where not already granted under a licence) please go to http://group.bmj.com/group/rights-licensing/permissions

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Contour funnel plots showing the publication bias in randomised controlled trials (RCTs) and observational studies. Publication bias was identified for observational studies as reflected by the asymmetrically distributed component studies.