Librarians and information scientists have been evaluating journals for at least 75 years. Gross and Gross conducted a classic study of citation patterns in the '20s. Others, including Estelle Brodman with her studies in the '40s of physiology journals and subsequent reviews of the process, followed this lead. However, the advent of the Thomson Reuters citation indexes made it possible to do computer-compiled statistical reports not only on the output of journals but also in terms of citation frequency. And in the '60s we invented the journal "impact factor." After using journal statistical data in-house to compile the Science Citation Index® (SCI®) for many years, Thomson Reuters began to publish Journal Citation Reports® (JCR®) in 1975 as part of the SCI and the Social Sciences Citation Index® (SSCI®).
Informed and careful use of these impact data is essential. Users may be tempted to jump to ill-formed conclusions based on impact factor statistics unless several caveats are considered.