1. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2651880/?tool=pubmed
  2. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2330043/?tool=pmcentrez
  3. http://www.benthamscience.com/open/toaltmedj/openaccess2.htm
  4. http://www.ncbi.nlm.nih.gov/pubmed/2912010
  5. http://www.jissn.com/content/7/1/5