Eugene Garfield, the creator of the journal Impact Factor, wrote an article outlining its history in 2006.
Like the Impact Factor, the Eigenfactor Score and Article Influence Score use ISI Web of Knowledge citation data to assess and track the influence of a journal in relation to other journals in the same discipline.
The aggregate Impact Factor for a subject category is calculated the same way as the Impact Factor for a journal, but it takes into account the number of citations to all journals in the category and the number of articles from all journals in the category.
An aggregate Impact Factor of 1.0 means that that, on average, the articles in the subject category published one or two years ago have been cited one time. The median Impact Factor is the median value of all journal Impact Factors in the subject category.
The journal Impact Factor is the average number of times articles from the journal published in the past two years have been cited in the Journal Citation Reports (JCR) year.
The Impact Factor is calculated by dividing the number of citations in the JCR year by the total number of articles published in the two previous years. An Impact Factor of 1.0 means that, on average, the articles published one or two year ago have been cited one time. An Impact Factor of 2.5 means that, on average, the articles published one or two year ago have been cited two and a half times. Citing articles may be from the same journal; most citing articles are from different journals.
The journal Impact Factor was developed by Eugene Garfield at the Institute for Scientific Information (ISI), now owned by Thomson Reuters.
Impact Factor uses Thomson Reuters (ISI Web of Knowledge) citation data.
“ISI coverage tends to be excellent in physics, chemistry, molecular biology, and biochemistry, biological sciences related to humans and clinical medicine; good, yet not excellent, in applied and engineering sciences, biological sciences related to animals and plants, geosciences, mathematics, psychology and other social sciences related to medicine and health; and moderate in other social sciences including sociology, political science, anthropology and educations sciences, and particularly in humanities (Moed, 2005, p. 3)
Likewise, disciplines that rely on books and conferences to share scholarly research may be at a disadvantage (Moed, 2005, p. 3)
Moed. H.F. (2005). Citation analysis in research evaluation. Dordecht, The Netherlands: Springer
The 5-year journal Impact Factor is the average number of times articles from the journal published in the past five years have been cited in the JCR year. It is caclulated by dividing the number of citations in the JCR year by the total number of articles published in the five previous years.
The 5-year Impact Factor is available only in JCR 2007 and subsequent years.
For comparing journals specializing in cutting-edge research, the immediacy index can provide a useful perspective.
The Immediacy Index is the average number of times an article is cited in the year it is published.
The Immediacy Index is calculated by dividing the number of citations to articles published in a given year by the number of articles published in that year.