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. 2019 Mar 30;9(3):e023436. doi: 10.1136/bmjopen-2018-023436

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© Author(s) (or their employer(s)) 2019. Re-use permitted under CC BY-NC. No commercial re-use. See rights and permissions. Published by BMJ.

This is an open access article distributed in accordance with the Creative Commons Attribution Non Commercial (CC BY-NC 4.0) license, which permits others to distribute, remix, adapt, build upon this work non-commercially, and license their derivative works on different terms, provided the original work is properly cited, appropriate credit is given, any changes made indicated, and the use is non-commercial. See: http://creativecommons.org/licenses/by-nc/4.0/.

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Test power as a function of effect size and intraclass correlation coefficient (ICC). The ICC describes the relative similarity of participants within units and is typically as low as 0.05 or 0.01⁷²; we have added the pessimistic scenario of ICC=0.10 for completeness only.