The following are links to sites discussing study designs:
http://galton.uchicago.edu/~thisted/courses/315/lectures/0297.pdf
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2689572/pdf/Dtsch_Arztebl_Int-106-0262.pdf
http://www.surgicalcriticalcare.net/Statistics/design.pdf
http://www.cardiorhythm.com/2009/pdf/20/67.pdf
http://www.cof.orst.edu/cof/teach/fs599_08/Readings/Bryan%20F.J.%20Manly,%20Chtr.%201.pdf