Journal impact factor: a brief review

All citation studies should be normalized to take into account variables such as field, or discipline, and citation practices. Citation density and half-life are also important variables. The citation density (mean number of references cited per article) would be significantly lower for a mathematics article than for a life sciences article. The half-life (number of years, going back from the current year, that cover 50% of the citations in the current year to the journal) of a physiology journal would be longer than that of a journal of molecular biology or astronomy. The impact factors currently reported by the Institute for Scientific Information in Journal Citation Reports (JCR) may not provide a complete enough picture for slower changing fields with longer half-lives. Nevertheless, when journals are studied within disciplinary categories, the rankings based on 1-, 7- or 15-year impact factors do not differ significantly, as was recently reported in The Scientist.3,4 In the first report the top 100 journals with the highest impact factors were compared;3 in the second report the next 100 journals were compared.4 When journals were studied across fields, the ranking for physiology journals as a group improved significantly as the number of years increased, but the rankings within the group did not. Hansen and Henrikson5 reported "good agreement between the journal impact factor and the overall [cumulative] citation frequency of papers on clinical physiology and nuclear medicine." There is a widespread but mistaken belief that the size of the scientific community that a journal serves affects the journal's impact. This assumption overlooks the fact that the larger the author and article pool for citing, the larger the number of published articles to share those citations. Many articles in large fields are not well cited, whereas those in small fields may have unusual impact. Therefore, the key determinants in impact are not the number of authors or articles in the field but, rather, the mean number of citations per article (density) and the half-life or immediacy of citations to a given journal. This distinction was discussed many years ago in an essay on "Garfield's constant".6 The size of a field, however, will determine the number of "super-cited" papers. While a few famous methodology papers achieve a high threshold of citation, thousands of other methodology papers do not achieve this distinction. The use of journal impact factors instead of actual