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. 2023 Apr 18;13(4):e069553. doi: 10.1136/bmjopen-2022-069553

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© Author(s) (or their employer(s)) 2023. Re-use permitted under CC BY-NC. No commercial re-use. See rights and permissions. Published by BMJ.

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Percentage of retrospectively registered trials reporting retrospective registration transparently in the publication over time (per study publication year). Generalised additive model smoother laid over (blue) with 95% CI. Bubble sizes indicate the number of trials per year included in the dataset. Starting in 2013, some authors begin to report retrospective registration. 15% of publications of retrospectively registered trials from 2020 transparently report retrospective registration. Four trials were published before 2009—in all those cases, the study completion dates provided in the registry were after 2009. Study start dates were before 2005 and studies were registered in 2005 (3/4) or later (1/4).