1. Oxford Centre for Evidence-Based Medicine 2011 Levels of Evidence [Internet],2011
2. Evidence-based medicine must be;La Caze;J Med Philos,2009
3. Benefits and risks of drug treatments: how to combine the best evidence on benefits with the best data about adverse effects;Vandenbroucke;JAMA,2008
4. Why we need observational studies to evaluate the effectiveness of health care;Black;BMJ,1996
5. Assessment of the external validity of randomised controlled trials;Rothwell,2007