Journal impact factors are based upon calculations of the frequency of citation for each article published in a journal. Although there are several sources for this information, the most comprehensive is that provided by Journal Citation Reports®, Science Edition, 2010. Journals with higher impact factors are read and cited more frequently than lower ranking journals.
To put this information into context, the New England Journal of Medicine is a premier publication of all aspects of medicine with an impact factor of 52.589. In the sub-specialty of gastroenterology and hepatology, Gastroenterology is the highest ranked at 12.432.