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The idea of impact factors was originally published by Dr. Eugene Garfield in a 1955 article in Science. Dr. Garfield and Irving H. Sher created journal impact factors in the early 60’s as a method for selecting journals for Science Citation Index at Thomson ISI (now Clarivate Analytics). Dr. Garfield indicated the need to compare journals regardless of their “size or citation frequency.” The impact factor quickly became the standard for evaluating the importance of journals in humanities and the sciences. Eventually, it became possible to search for impact factors through Journal Citation Reports, a stand-alone database. In 2005, Dr. Garfield presented a full history of the process at: http://garfield.library.upenn.edu/papers/jifchicago2005.pdf
Over the years, alternatives and other options to gauge impact of journals have evolved. In 2018 the UAMS Library canceled the subscription to Journal Citation Reports. Links to information and options are below.
The Centre for Science and Technology Studies provides various impact metrics calculated on SCOPUS data for scientific journals including the SNIP and stability indicators. Details on their methodology is at: http://www.journalindicators.com/methodology
Developed at the University of Washington, this free resource provides Eigenfactor and Article Influence scores. "The algorithms use the structure of the entire network (instead of purely local citation information) to evaluate the importance of each journal." Scores are based on 5-years of data. "Under an agreement with Thomson-Reuters, we are able to bring you the Eigenfactor metrics here at Eigenfactor.org free of charge. Scores will appear here annually, six months after they are released in the Thomson-Reuters Journal Citation Reports."
This free portal presents indicators on journals and country rankings pulled from the SCOPUS database (Elsevier). Data on journals and/or countries can be combined or analyzed separately for each. Journals can be grouped by 313 subject categories. Citation data is pulled from "over 34,100 titles from more than 5,000 international publishers and country performance metrics from 239 countries worldwide".
GS provides metrics for each journal as a way to rank them. It is searchable by major categories or by individual journal title. Link defaults to top journals in English. Top journals are also available for 11 other languages by clicking the drop down menu arrow. To find metrics for individual journals, see step-by-step instructions below.