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. 2018 Jun 22;18:61. doi: 10.1186/s12874-018-0513-y

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© The Author(s) 2018

Open Access This 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 — Relative bias (rB) of the methods. a: True model b: conventional model; c : 2SRI with instrument pr; d: GMM with instrument pr. Low, Medium and High indicate the corresponding level of confounding and the instrument strength grows from a, b, c, d sequence to the next (from left to right)