The analysis is published in Ann Intern Med May issue. Large clinical trials are taken into account. More than half of the studies are conducted in USA. This shows that we are still having big problems with transparency and missing important data.
67 preregistered but unreported trials are identified with a median enrollment of 765 (range, 511–11,000), which are unreported for a median of 9 years after the end of the clinical trial.
Ann Intern Med. 2019 May 7. doi: 10.7326/M19-0440. Lost Evidence From Registered Large Long-Unpublished Randomized Controlled Trials: A Survey. Tatsioni A, Karassa FB, Goodman SN, Zarin DA, Fanelli D, Ioannidis JPA