Impact factor is a measure of the frequency with which the average article in a journal has been cited in a particular year or period.
Journal Citation Reports (JCR) is a web-based widely used research tool that allows users to evaluate and compare journals using citation data drawn from approximately 12,000 scholarly and technical journals and conference proceedings from more than 3,300 publishers in over 60 countries.
The annual JCR impact factor is a ratio between citations and recent citable items published. Thus, the impact factor of a journal is calculated by dividing the number of current year citations to the source items published in that journal during the previous two year.
|Calculation for journal impact factor.|
|A= total cites in 1992|
|B= 1992 cites to articles published in 1990-91 (this is a subset of A)|
|C= number of articles published in 1990-91|
|D= B/C = 1992 impact factor|
The impact factor is useful in clarifying the significance of absolute (or total) citation frequencies. It eliminates some of the bias of such counts which favor large journals over small ones, or frequently issued journals over less frequently issued ones, and of older journals over newer ones. Particularly in the latter case such journals have a larger citable body of literature than smaller or younger journals. All things being equal, the larger the number of previously published articles, the more often a journal will be cited.