The impact factor is a quantitative tool provided by Thomson Reuters, in its annual Journal Citation Reports, for ranking, evaluating, categorizing, and comparing journals. A journal’s impact factor is defined by Thomson Reuters as “a measure of the frequency with which the ‘average article’ in a journal has been cited in a particular year or period.”
To calculate a journal’s impact factor for a particular year, divide the number of citations for that year into the source items published in that journal for 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
See the Thomson Scientific Web site for additional information.