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1. Oxford Centre for Evidence-Based Medicine. Levels of evidence and grades of recommendation. Available from http://www.cebm.net/levels_of_evidence.asp

2. Straus SE, Richardson WS, Glasziou P, Haynes RB. Evidence-based medicine: how to practice and teach EBM, 3rd ed. Philadelphia: Elsevier Churchill Livingstone, 2005.

3. Gehlbach SH. Interpreting the medical literature, 3rd ed. New York: McGraw-Hill, 1993.