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What does it mean to be #2 in Impact?

by on ‎06-17-2010 01:17 PM

For many years, the journal CA-A Cancer Journal for Clinicians has posted the highest Journal Impact Factor in the JCR Science Edition.  It remains on the top of the general list this year (Journal Impact Factor=87.925), but a very unexpected journal emerges in the second spot: Acta Crystallographica – Section A, with a Journal Impact Factor of 49.926 – over 20-fold higher than last year’s value.  


Stunning achievement, or flawed metric? 


I would argue that it is neither.  Rather, it is an opportunity to examine what the metrics in the JCR really mean. 


My first response to this fantastic Journal Impact Factor was to de-construct it.  Using Web of ScienceSM, I searched for the Publication CRYSTALLOGRAPHICA SECTION A , and the Year published=(2007 or 2008).  This allowed me to see the materials, and most of the citations, that contribute to the Journal Impact Factor calculation.  I retrieved 126 items, and ranked them by the Times Cited value.  The explanation was immediately apparent.  The 2008 article “A short history of SHELX “ by GM Sheldrick  was cited 6436 times (as of today, June 17, 2009).  The next most-cited article, “On the application of an experimental multipolar pseudo-atom library for accurate refinement of small-molecule and protein crystal structures”, shows only 28 citations. The Sheldrick article was clearly an unusual item.  Viewing the 6436 citing articles, and refining these results by the Publication year shows the 2391 citations in 2009 that defined the majority of the Journal Impact Factor numerator. So this article fully accounts for the meteoric rise in Journal Impact Factor. 


In this context then, what does the very high impact of this journal mean?  It is clear that the journal published an article that has had an impact in the field (4893 of the citing articles are in journals in the Crystallography category in Web of Science).  Such a marked impact, in fact, that it raised the journal to an unprecedented prominence in the literature as a whole.  This is really “impact” and the Journal Impact Factor reflects it.  This article and its citations will be considered in the Journal Impact Factor calculation this year and next year.  Without another, similarly important article in 2010, Acta Crystallographica – Section A is likely to return in 2011 to its prior Journal Impact Factor of between 1.5 and 2.5.  A valued journal for specialists in this field. 


Other indicators


It is also interesting to examine how the other metrics in the JCR support, and qualify the Journal Impact Factor.  The Cited journal graph (and the underlying data) show the unusual spike in the cited year 2008.  This quickly leads the user to the single-article explanation. 


When the 2008 data were released, there were no comments on the fact that Acta Crystallographica – Section A posted an Immediacy Index of 49.556 – the highest value in the 2008 JCR - twice that of CA-A Cancer Journal for Clinicians and 4-fold higher than New England Journal of Medicine, the long-unchallenged leaders in both Journal Impact Factor and Immediacy Index.  The very high Journal Impact Factor published this year was anticipated by the 2008 Immediacy Index.  This year’s Immediacy index is 0.438, a 100-fold drop from last year, but very consistent with prior years, further emphasizing the unsual influence of the Sheldrick paper. 


Finally, the journal ranks much lower according to the Eigenfactor™ score, ranking 397 of the 6598 journals in the Science Edition of the JCR.  The Eigenfactor™ score is a reflection of the influence of a journal in the structure of the citation network as a whole.  Of course, any journal with an article receiving nearly 2400 citations in a single year will be influential – but since the Eigenfactor™ score is not scaled to the size of the journal, this influence is considered in the broader context of journals that regularly publish many articles with high level of citation. 


Impact, immediacy, influence – all are affected by this explosively cited article, but the differing magnitudes of the effects highlight the value of the multi-faceted view of journals that is provided by JCR data. 

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