In 2009 Thomson Reuters for the first time released the new 5-year journal Impact Factor in addition to the standard 2-year journal Impact Factor . The 5-year journal Impact Factor is the average number of times articles from the journal published in the past five years have been cited in the JCR (Journal Citation Report) year. It is calculated by dividing the number of citations in the JCR year by the total number of articles published in the five previous years.
The controversy around the Impact Factor tradition has not been able to deter the Impact Factor from rising to the most important quality assessment tool in scientific journal publishing. It has to be noted that the value of the Impact Factor cannot be compared among different scientific disciplines. For instance Microbiology journals have, on average, much higher Impact Factors than Mathematics or Engineering journals. The citation patterns in these disciplines are entirely different, therefore the numerical values of their Impact Factors also differ significantly and comparisons would not yield appropriate results.