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. 2018 Mar 17;8(3):e018959. doi: 10.1136/bmjopen-2017-018959

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© Article author(s) (or their employer(s) unless otherwise stated in the text of the article) 2018. All rights reserved. No commercial use is permitted unless otherwise expressly granted.

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Timeline of the Nineteen and Up study data collection. Because of the changes in protocols, participants from NU1 were all reapproached to complete the following NU2 or NU3 waves. The vast majority (92%) then completed NU2 or NU3. Despite an interval of several years, these data provide an opportunity to examine test–retest reliability of scores or compare collection methods (eg, self-report online vs telephone interview). In addition to providing insight into the validity of the measures, these data are important for twin modelling as the stability of a phenotype or diagnosis sets an upper limit for the heritability.