Journal Impact Factor (JIF) is the average number of times articles from the journal published in the past two years have been cited in the 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 works are from journals, proceedings, or books indexed by Web of Science.
A = total cites in 2015
B = 2015 cites to articles published in 2013-2014 (a subset of A)
C = number of articles published in 2013-2014
D = B/C = 2015 impact factor (published in June 2016)
JCR year is a calendar year from January to December. JIF metrics for the JCR year are published annually next June, that is, JIF data for 2015 were released in June 2016.
Citable Items include research articles, reviews, notes, and corrections/retractions. An item is classified as a review if it meets any of the following criteria:
Each citable item is manually coded. Other items include editorials, letters, news items, and meeting abstracts. These items are not counted in JCR calculations because they are not generally cited.
Self-Citation is a reference to an article from the same journal. Self-citations can make up a significant portion of the citations a journal gives and receives each year. Impact Factor Without Journal Self Cites is available in JCR.
Total Cites is the total number of times that a journal has been cited by any publication included in Web of Science in the JCR year. Cites are not coded manually, and not limited to original research articles or reviews. Each unique article-to-article link is counted as a citation.
(Adapted from Thomson Reuters).