The impact factor was devised by ‘Eugene Garfield’, the founder of the ‘Institute for Scientific Information’ (ISI), now part of ‘Thomson Reuters’. Impact factors are calculated yearly for those journals that are indexed in ‘Thomson Reuters’ ‘Journal Citation Reports’ (JCR). JCR provides quantitative tools for evaluating, categorizing and comparing journals. The impact factor (IF) is a measure of average number of citations to the articles published in science and social science journals. It is a measure of the frequency with which the average article in a journal has been cited in a given period of time. The annual JCR impact factor is a ratio between citations and recent citable items published. The impact factor for a journal is calculated based on a three-year period and can be considered to be the average number of times published papers are cited up to two years after publication. In general high impact factor journals are recognized as the most influential as compared to the other journals published in the same field. The IF is used to compare different journals within a certain field. The ‘Institute for Scientific Information’ indexes more than 11,000 science and social science journals.
A) Calculation of yearly impact factor based on three year period.
X=Total cites in 2012
Y= 2012 cites to articles published in 1907-11 (This is subset of X)
Z= Number of articles published in 1907-11
IF=Y/Z=2012 impact factor
Actual Calculation of impact factor in 2013
Cites in 2012 to articles published in:
2011 = 586
2010 = 596
Total = 1182
Number of articles published in:
2011 = 125
2010 = 122
Total = 247
Impact factor = cites to recent articles/number of recent articles = 1182/247=4.78
B) Calculation of five year impact factor
X= Citations in 2012 over the 2007-2011
Y= Articles published over the years 2007-2011
Z= X/Y = Five year impact factor
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