1. Births: final data for 2005;Martin;Natl Vital Stat Rep,2007
2. BMJ clinical evidence systematic reviews 2007 http://clinicalevidence.bmj.com/ceweb/about/systematic_reviews.jsp
3. Scottish Intercollegiate Guidelines Network Critical appraisal: notes and checklists 2007 www.sign.ac.uk/methodology/checklists.html
4. Oxford Centre for Evidence-Based Medicine Levels of Evidence: Number Needed to Treat (NNT) Centre for Evidence-Based Medicine www.cebm.net/index.aspx?o=1044