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This paper is a very valuable addition to the literature on clinical trial transparency. It illustrates once again that self-regulation by the medical research sector does not work.
One limitation is the data provided in the supplementary appendix. The authors could have listed the full information gathered on each trial, including trial number (where available), name of PI, and sponsor name. As the recent STAT investigation into unreported trials (https://www.statnews.com/2018/01/09/clinical-trials-reporting-nih/) has shown, making performance transparent in and of itself can drive the subsequent adoption of best practices, which is in the interests of patients and the research community alike.
Even if confidentiality agreements precluded the identification of trials, it may have been possible to include more granular data without enabling the re-identification of specific trials. For example, many countries and funders have laws, policies and regulations that make prospective trial registration compulsory (https://docs.wixstatic.com/ugd/01f35d_def0082121a648529220e1d56df4b50a.pdf).
Knowing in which countries (rather than aggregated global regions) the unregistered and retrospectively registered trials were conducted, which funders (rather than aggregated funder types) funded them,...
Knowing in which countries (rather than aggregated global regions) the unregistered and retrospectively registered trials were conducted, which funders (rather than aggregated funder types) funded them, and which individual journals published them, would immediately flag up systems that are currently not working as intended.
This would provide a useful entry point for other researchers to examine why current safeguards are not working in these specific countries, funding agencies, and journals. It would also alert those working in the responsible entities that their current systems are not working as intended, and thus enable them to take corrective action.