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. 2020 Jan 3;18:1. doi: 10.1186/s12916-019-1443-1

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© The Author(s). 2020

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. 1 — Study selection. a PRISMA flow chart depicting study screening and selection; approximately 150 studies were received from the Bodleian Library/British National Library. b Data availability plotted by year (x-axis) and country (y‐axis), grouped by region. The number of studies for each country‐year is depicted by the size of the point