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. 2018 Oct 29;19:589. doi: 10.1186/s13063-018-2983-y

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© The Author(s). 2018

Open AccessThis 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 — Registered trials confirmed as using qualitative methods by registry by year. ICTRP International Clinical Trials Registry Platform, ISRCTN International Standard Randomised Controlled Trial Number Registry, WHO World Health Organisation