Per Thomson Reuters, the JCR provides quantitative tools for ranking, evaluating, categorizing, and comparing journals. The impact factor is one of these; it is a measure of the frequency with which the "average article" in a journal has been cited in a particular year or period. 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 years
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
