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. 2016 Aug 22;16(1):103. doi: 10.1186/s12874-016-0197-0

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© The Author(s). 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. 1 — Illustration of the timeline of the case-crossover design, shown for four hypothetical subjects with respect to one risk factor. The dark gray area just prior to the event of interest marks the case period, the light gray area marks the reference period. Exposure to the risk factor is marked by “x”. Note that subjects 1 and 2 are discordant (differing exposures between case and reference periods), whereas subjects 3 and 4 are concordant (same exposures in case and reference periods)