The history and meaning of the journal impact factor.

IFIRST MENTIONED THE IDEA OF AN IMPACT FACTOR IN Science in 1955. With support from the National Institutes of Health, the experimental Genetics Citation Index was published, and that led to the 1961 publication of the Science Citation Index. Irving H. Sher and I created the journal impact factor to help select additional source journals. To do this we simply re-sorted the author citation index into the journal citation index. From this simple exercise, we learned that initially a core group of large and highly cited journals needed to be covered in the new Science Citation Index (SCI). Consider that, in 2004, the Journal of Biological Chemistry published 6500 articles, whereas articles from the Proceedings of the National Academy of Sciences were cited more than 300 000 times that year. Smaller journals might not be selected if we rely solely on publication count, so we created the journal impact factor (JIF). The TABLE provides a selective list of journals ranked by impact factor for 2004. The Table also includes the total number of articles published in 2004, the total number of articles published in 2002 plus 2003 (the JIF denominator), the citations to everything published in 2002 plus 2003 (the JIF numerator), and the total citations in 2004 for all articles ever published in a given journal. Sorting by impact factor allows for the inclusion of many small (in terms of total number of articles published) but influential journals. Obviously, sorting by total citations or other provided data would result in a different ranking. The term “impact factor” has gradually evolved to describe both journal and author impact. Journal impact factors generally involve relatively large populations of articles and citations. Individual authors generally produce smaller numbers of articles, although some have published a phenomenal number. For example, transplant surgeon Tom Starzl has coauthored more than 2000 articles, while Carl Djerassi, inventor of the modern oral contraceptive, has published more than 1300. Even before the Journal Citation Reports (JCR) appeared, we sampled the 1969 SCI to create the first published ranking by impact factor. Today, the JCR includes every journal citation in more than 5000 journals—about 15 million citations from 1 million source items per year. The precision of impact factors is questionable, but reporting to 3 decimal places reduces the number of journals with the identical impact rank. However, it matters very little whether, for example, the impact of JAMA is quoted as 24.8 rather than 24.831. A journal’s impact factor is based on 2 elements: the numerator, which is the number of citations in the current year to items published in the previous 2 years, and the denominator, which is the number of substantive articles and reviews published in the same 2 years. The impact factor could just as easily be based on the previous year’s articles alone, which would give even greater weight to rapidly changing fields. An impact factor could also take into account longer periods of citations and sources, but then the measure would be less current.