How is the Journal Impact Factor calculated?

 How is the Journal Impact Factor calculated?

The Journal Impact Factor assesses journals catalogued within Thomson Reuters ‘Web of Knowledge’. It is a measure of how often an article in a particular journal has been cited, on average, per year, therefore giving the journal’s relative influence or impact compared to journals in the same area.

The Impact Factor (IF) is based on the number of citations (A) in the current year, compared with items published in the previous 2 years, and the number of articles (B) published in the same two years:

This calculation is repeated each year, so that the changing impact factor for a journal can be observed over time