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.
The journal Impact Factor was developed by Eugene Garfield at the Institute for Scientific Information (ISI), now owned by Thomson Reuters.
Garfeild wrote an article outlining its history and meaning in 2006.
Impact Factor uses Thomson Reuters (ISI Web of Knowledge) citation data.
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.