I have just read in a news@nature (http://www.nature.com/news/2004/040726/full/040726-16.html) a column by Phillip Ball on the paper of Sidney Redner (http://xxx.arxiv.org/abs/physics/0407137), who have studied the statistics of citations from all Physical Review journals for the 110-year period 1893 until 2003. Following Ball, "The list suggests not only that Kohn's Nobel Prize was absurdly overdue, but also that John C. Slater (another pioneer of quantum solid-state theory) was unjustly overlooked". I think it is very interesting to analyze the function of the "impact factors" (number of citations, age of citations and so on) in the development of modern science. First, because of they suggest where important advances are being produced. Second, because they signal trends. In general terms, a good paper is cited and a bad paper is unnoticed. But, What about the side effects? How many good lines are being aborted because an excessive worry about such impact factors?