As far as a I know, there are no published studies available to answer that question beyond the obvious - you are missing some. It isn't just the actual titles indexed but also how well an interface to a database works in helping one retrieve what is indexed in it. And also the search methods one uses to locate the RCTs. Not all RCTs are actually indexed as such in Medline or any database for that matter. There are some published studies that show that each database doesn't cover everything. The first one listed below might be the most informative but it is behind a paywall for me.
Suarez-Almazor, M. E., E. Belseck, J. Homik, M. Dorgan, and C. Ramos-Remus. 2000.
Identifying clinical trials in the medical literature with electronic databases:
MEDLINE alone is not enough. Controlled Clinical Trials 21 (5):476–87.
http://www.ncbi.nlm.nih.gov/pubmed/11018564Gavel, Y., and L. Iselid. 2008. Web of Science and Scopus: A journal title overlap
study. Online Information Review 32 (1):8–21.
http://tinyurl.com/ldbtlkr
Svetla Baykoucheva (2010): Selecting a Database for Drug Literature Retrieval: A
Comparison of MEDLINE, Scopus, and Web of Science, Science & Technology Libraries, 29:4, 276-288
http://drum.lib.umd.edu/bitstream/1903/11411/1/BaykouchevaMedlineScopusWoS.pdf
Jacso, P. 2005. As we may search—Comparison of major features of the Web
of Science, Scopus, and Google Scholar citation-based and citation-enhanced
databases. Current Science 89 (9):1537–47.
