Why is observational studies still necessary?

http://www.bmj.com/cgi/content/full/bmj%3B312/7040/1215?eaf

Randomized controlled trials

Zelen, Wennberg, Intent to treat design

Bias

Bias and causal associations in observational research

Reliability and Validity

http://webs.wofford.edu/reidak/Psych%20200/Reliability%20and%20validity.pdf