1. Wikipedia. Controlled experiments. [updated 2008 June 19; cited 2008 June 19]. Available from URL: http://en.wikipedia.org/wiki/Controlled_experiment.
2. Guide to clinical preventive services. An assessment of the effectiveness of 169 interventions. Report of the U.S. Preventive Services;U.S. Preventive Services Task Force,1989
3. Observational studies;Rosenbaum,2002
4. The Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) statement: guidelines for reporting observational studies;von Elm;Ann Intern Med,2007
5. Risk adjustment for measuring healthcare outcomes,2003