INTRODUCTION
PubMed is a resource tool maintained by the National Center for Biotechnology Information (NCBI) for the National Library of Medicine (NLM). As the screen capture of the home page above tells us, it contains over 22 million citations and abstracts of biomedical literature from various sources worldwide (foremost of which is MEDLINE, or the Medical Literature Analysis and Retrieval System Online), and it remains a free tool for all users, although some of the citations may link to full-text articles that must be purchased through other vendors.