The Impact Factor, calculated and published by the ISI Web of Knowledge, is tossed around lately as a means of rating the significance of a journal. It was first thought up back in the 1950s and started to gain traction in the 1970s as a regular assessment tool. Over the last 10 years, it has now become, in some respects, the dominant indicator of a journal’s worth.
Therefore, it’s useful to spend some time discussing this number and what it means for our field. Because I try to keep these blog posts to under 500 words, I am going to have a series of articles about it. This first post is to give you the formula and the pertinent numbers for JGR-Space Physics.
The Impact Factor follows a very specific and relatively simple formula:
Year X Impact Factor =
Citations in Year X to Papers in Years X-1 and X-2
Papers Published in the Journal in Years X-1 and X-2
Note that the numerator is for all journals across all disciplines, not just citations within the journal to papers published in that journal. Those are called self cites, and they are usually a small fraction of the total cites. The denominator is anything with a unique digital object identifier (DOI) in that journal those years. This total includes not only research articles and reviews but also editorials, forewords, and corrections, with the latter grouping usually receiving far fewer citations than the former.
ISI does not separate out the different titles within the JGR name umbrella, so all 7 sections of the journal contribute to the same Impact Factor value. For 2012, ISI has these numbers for the Journal of Geophysical Research:
Number of items published in 2011: 2707
Number of items published in 2010: 2999
Cites in 2012 to items published in 2011: 7422
Cites in 2012 to items published in 2010: 10688
2012 Impact Factor = 18110 / 5706 = 3.174
Yes, you are reading it correctly…only 4 numbers go into the calculation of Impact Factor. In 2012, JGR received over 160,000 cites to its articles, but only a little over 10,000 of those were to papers published in the two years included in the Impact Factor calculation.
I will have other posts on the history of the Impact Factor, values for other space physics journals, similar factors and indices, and a more thorough breakdown of citations in JGR. If you have suggestions about posts on this topic, then please let me know.