This is a nice example of mining the existing medical literature to discover trends and attitudes about research. The authors take too dim a view, in my opinion, of what observational studies can show, but the statistics are still interesting to follow.
Vinay Prasad, Joel Jorgenson, John P.A. Ioannidis, Adam Cifu. Observational studies often make clinical practice recommendations: an empirical evaluation of authors' attitudes. Journal of Clinical Epidemiology. Volume 66, Issue 4, April 2013, Pages 361-366.e4. DOI: 10.1016/j.jclinepi.2012.11.005. Available in PDF format.