1. Tips and tricks: The role of systematic reviews;O'Mathúna;International Journal of Nursing Practice,2010
2. Oxford Centre for Evidence-based Medicine Levels of evidence 2009 http://www.cebm.net/index.aspx?o=1025
3. Preferred reporting items for systematic reviews and meta-analyses: The PRISMA statement;Moher;PLoS Medicine,2009
4. Burls A What is critical appraisal? 2009 http://www.medicine.ox.ac.uk/bandolier/painres/download/whatis/What_is_critical_appraisal.pdf