This month the Forum begins a new series on journal impact factors. The series will address what they are, how they are calculated and applied, and offer caveats for using IF data wisely.

Quick history of citation analysis

In the 1920's information scientists and librarians became interested in studying the phenomenon of citation patterns (i.e., who was being cited and where they were published) in the journal literature. Interest in citation analysis inspired the creation of the Institute for Scientific Information (ISI)® citation indexes. The ISI citation indexes allowed for computer-compiled statistical reports not only on the output of journals, but also on citation frequency. The concept of impact factor was created in the 1960's and ISI began to publish Journal Citation Reports (JCR)® in 1975 as a part of the Science Citation Index® and the Social Sciences Citation Index®. JCR® uses impact factors, along with other criteria, to compare and rank journals.

So what is the impact factor?

A journal's impact factor is a measure of the frequency with which the "average article" in that journal has been cited in a particular year or period. Citations are considered as important indicators of how frequently current researchers are using the journal literature.

The types of articles that are listed in the JCR® include:

• Only original research articles
• Review articles
• Proceedings and technical notes

Editorials, letters, news items, and meeting abstracts are not counted as source items because they are not generally cited.