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. 2016 Jun 1;17:270. doi: 10.1186/s13063-016-1399-9

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© Hall et al. 2016

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. 3 — Pattern of usage over time for selected primary outcome instruments. Note that the identification of studies categorised as ‘pre-July 2006’ may not be representative as many records relating to this period would have been excluded according to our search criteria