I first mentioned the idea of an impact factor in Science in 1955.1 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.2 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,3 so we created the journal impact factor (JIF).
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