The Krafty Librarian posts about Problems with ISI’s Impact Factor Data. The editors of the Journal of Cell Biology and the Journal of Experimental Medicine have written an editorial reporting their inability to verify published impact factors using data provided by ISI, and they’re questioning the validity of the dataset.
Rossner M, Van Epps H, Hill H. Show me the data. J Cell Biol 2007; 179 (6): 1091-1092. Published online December 17 2007, doi:10.1083/jcb.200711140. http://www.jcb.org/cgi/content/full/179/6/1091