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. 2018 Jan 5;18:3. doi: 10.1186/s12874-017-0467-5

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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. 1 — Participation rates for each dispatch arms. * P-values of participation to one or two components are shown. ^a + = presence of pre-notification; − = absence of pre-notification. ^b + = long invitation letter; − = short invitation letter, ^c + = long questionnaire; − = short questionnaire. ^d + = presence of reminder; − = absence of reminder