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How Reliable Are Impact Factors?

The impact factor is a frequently used measure to assess a journal’s importance in its scientific field. For a certain journal and year, it is calculated as the number of citations in this year referring to articles published in the journal within the last two years divided by the total number of articles published within the last two years. The validity of this formula is put in question by looking at the numbers for 2009.

According to Thomson Reuters’ Journal Citation Report 2009, the second highest ranking journal in 2009 is Acta Crystallographica – Section A with an astonishing impact factor of 49.926. This number is only surpassed by that of CA-A Cancer Journal for Clinicians (87.925) and much higher than those of Nature, Science, and the New England Journal of Medicine which all have been leading in impact factors for several years. Furthermore, Acta Crystallographica – Section A had an impact factor in the region 1.5 to 2.5 in preceding years. What is the reason for this drastic change?

The reason becomes immediately apparent when looking at the journal’s citations in detail: The article A short history of SHELX by G. M. Sheldrick, published in January 2008, was cited 6436 times till June 2009 whereas the next most-cited article shows only 28 citations. This means that one single article fully accounts for the dramatic increase of the journal’s impact factor. And as this crucial article will be considered in the calculation of the impact factor only in 2009 and 2010, the impact factor of Acta Crystallographica – Section A is likely to drop to its prior value of 1.5 to 2.5 in 2011. Regarding these numbers it is more than adequat to raise the question if the impact factor really measures a journal’s scientific importance.

Read http://www.the-scientist.com/blog/display/57500/

Read http://community.thomsonreuters.com/t5/Citation-Impact-Center/What-does-it-mean-to-be-2-in-Impact/ba-p/11386