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The impact factor was first described in 1955 by Dr. Eugene Garfield 1–4 and was used in the early 1960s to help select journals for what would evolve to become the Science Citation Index 1. The Science Citation Index, a commercial property of the Institute of Scientific Information (Philadelphia, Pennsylvania) 5–7, is used to generate the Journal Citation Reports, produced annually. The Journal Citation Reports list the total number of citations of materials published in a select index of journals during the preceding year.
The Institute of Scientific Information also produces a series of other commercial products of value to researchers and academics, including several other citation indexes (e.g., Social Sciences Citation Index, Arts and Humanities Citation Index, and Web of Science), current awareness products (e.g., Current Concepts Connect), and popular information management tools (e.g., EndNote and Reference Manager).
Since the first publication of the Journal Citation Reports in 1972 8, individuals and institutions have been trying to rank journals with use of the data provided in those Reports. However, direct comparison between journals on the basis of the total number of citations alone is an inappropriate measure influenced by a number of factors, such as journal format and content, appropriateness of article classification, and discipline-specific citation tendencies. Thus, a relative adjusted score was sought to allow direct quantitative comparison. Garfield's previously described impact factor was loosely adopted to suit this purpose in the late 1970s, but it did not come into true vogue until two decades later (the late 1990s) 3.
The original intention for the use of the impact factor was to allow comparison between the citation rates of journals 1,6,9. This has proven invaluable for researchers and librarians in the selection and management of journals 7,10. The application of this tool evolved into a means with which to assess the quality of the journals themselves 9,11, on the basis of the premise that a higher rate of citation indicated higher journal quality. Furthermore, the misuse of this calculation has, in recent years, widened to include evaluation of the quality of individual papers and even individual authors 1,6,9,12,13. There is evidence to suggest that some academic assessment committees and institutional promotion, tenure, and funding bodies have also adopted this tool for evaluation of individual researchers or research groups 3,5,8,10,11,14,15, although just how widespread this practice is remains unclear. A fundamental understanding of how the impact factor is derived highlights how inappropriate these applications are. Those who use it in such a manner clearly fail to either understand its true derivation or appreciate the original intended use 1. The purpose of this paper is to introduce the impact factor to those not intimately familiar with it and to discuss some of the biases and limitations of its application.
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